Season 1
In 2000, then 11-year-old Barry witnessed his mother being killed by a supernatural apparition, with himself being inexplicably taken out of the house at the time. Barry's father Henry was then arrested as the alleged murderer. Barry's testimony was not believed at the time, and he came into the family of Detective Joe West as a foster child.
Fourteen years later, Barry is working as a forensic investigator with the Central City Police Department, searching for his mother's real killer on the side. Shortly after returning from Starling City, however, he is struck by a strange lightning bolt while an explosion at S.T.A.R. Labs' particle accelerator hurls a wave of dark matter through the city. Barry falls into a coma and awakens nine months later with superpowers. He is now a speedster and able to move at lightning speed, has very fast physical regeneration, and several other abilities that develop over time.
The enthusiastic Barry, with the help of S. T.A.R. Labs employees Cisco Ramon and Dr. Caitlin Snow, who lost her fiancé Ronnie in the explosion, starts acting as the superhero Flash and capturing other people who also gained different superhuman abilities in the explosion (meta-beings) and commit crimes with them. Dr. Harrison Wells, the genius owner and head of S.T.A.R. Labs, becomes his mentor. Some of the meta-beings are imprisoned in the remains of the particle accelerator, which has been converted into a special prison.
Barry's foster father Joe, with whom he has a very good and intimate relationship, is initially not thrilled with his "exploits" but later becomes an important ally for the team as well. Iris, Joe's daughter and Barry's best friend, is excited about the "Red Flash" and starts writing a blog about it. Barry, who has been secretly in love with her for years, tries unsuccessfully several times to get her to stop in order to protect her (and him). Her enthusiasm for the Flash also causes tension with her boyfriend Eddie Thawne, Joe's young partner on the police force, whom she has been dating since Barry's accident.
Felicity Smoak (from the series Arrow), comes to town several times and also helps the team fight the crook Leonard Snart, aka Captain Cold, who becomes one of the most dangerous opponents. In addition, Barry finds that if he runs fast enough, he can travel through time: In an accidental time jump, he travels back a day and prevents the destruction of the city, but at the same time finds that he has inadvertently altered the timeline in other ways as well.
Gradually, Joe develops suspicions about Wells and begins to investigate. It becomes clear early on that Wells is not confined to a wheelchair since the accident, as initially shown, and that he is hiding something. Joe finds out that the explosion of the particle accelerator was planned by Wells so that Barry would get his abilities. It also becomes clear that Wells is the mysterious "man in the yellow suit", the speedster who murdered Barry's mother at the time.
The team gets on Wells' trail and finds his secret room with the yellow suit and the supercomputer Gideon, which Barry says he developed. At the same time it becomes clear that Wells has been spying on the whole team all along and thus knew about the investigation against him all along. Now he unleashes the mutant supergorilla Grodd on the city and reactivates the particle accelerator.
It turns out that Wells' real name is Eobard Thawne and he's from the 22nd century. He had traveled back in time to kill young Barry so that Flash, his nemesis, would never exist. However, the latter was saved by Barry's future self, whereupon Eobard killed Barry's mother out of frustration, thus ensuring that the traumatized Barry would never become the Flash. However, by altering the future in this way, he forfeited most of his powers and became trapped in the year 2000, after which he killed the real Harrison Wells and assumed his form and identity in order to complete the particle accelerator (which he said wasn't supposed to be ready until 2020, so Barry wouldn't have gotten his powers until then in the original timeline) sooner and turn Barry into the Flash so that he could return him to his own time.
Barry is able to stop Thawne and imprison him with the help of Oliver Queen, aka Arrow, and Caitlin's fiancé Ronnie, who is alive after all and has merged with scientist Prof. Martin Stein to form the meta-being Firestorm. Thawne reveals that he is a descendant of Iris' friend Eddie, and makes Barry an offer: the particle accelerator will serve as a time machine. If Barry releases Thawne, he will explain to him how to create a wormhole through which he can travel to the past and save his mother; Thawne, in turn, wants to use the same wormhole to travel back to the future.
Barry seemingly accepts the offer, though doing so would prevent him from ever becoming Flash, and could also greatly alter the course of history. In the past, Barry arrives just as Thawne and his future self are fighting. Future Barry gestures to warn his younger self not to save his mother; she is stabbed by Thawne. Barry then says goodbye to his mother before she succumbs to her injuries.
Back in the present, Barry now wants to stop the surprised Thawne before he gets back to the future. Thawne gets the upper hand in the ensuing fight and is about to kill the Flash when Eddie shoots himself so that Eobard Thawne will never be born. He then transforms back into his real self and subsequently dissolves, while Eddie dies. Simultaneously, the time anomaly thus created creates a black hole over the city, threatening to destroy it and the entire planet. The season ends with Barry trying to close the black hole with the help of his speed.
Season 2
Six months after the final events of the first season, i.e. after the Singularity, Flash is honored as a hero of Central City, even though Barry doesn't feel like a hero because of Eddie and Ronnie's deaths: Ronnie and Professor Stein had flown into the Singularity as a Firestorm and separated inside it, neutralizing it with the energy released in the process; Barry was able to save the falling Professor Stein, but Ronnie has since disappeared.
Jay Garrick, himself a speedster from a parallel universe called Earth 2 (while Barry's home is Earth 1), arrives on Earth 1 through one of the many rifts between the two dimensions caused by the Singularity and seeks out Barry to warn Team Flash about another speedster from Earth 2, named Zoom. Zoom is trying to steal the speed of all speedsters from other universes. Thus, he has already taken away Jay's powers. Later, Harrison Wells of Earth 2, whom they call simply "Harry" to avoid being constantly reminded of the wrong Wells (a.k.a. Eobard Thawne), also joins Team Flash in Earth 1 to stop Zoom.
Meanwhile, Joe and Iris struggle to come to terms with their family's painful past, especially after the arrival of Iris' mother and Iris' brother Wally, while Caitlin Snow must cope with losing her husband Ronnie a second time and now for good. In the meantime, a new mate must be found for Professor Stein to merge into Firestorm, or else he is in danger of dying. To that end, they find auto mechanic Jefferson "Jax" Jackson, who after some reluctance agrees to act as Firestorm's partner. Meanwhile, Zoom sends meta-beings through the many dimensional rifts from Earth 2 to Earth 1, which are meant to kill Barry but fail. One by one, Team Flash closes the dimensional rifts to the parallel world until only one remains. Furthermore, it turns out that Cisco has also been affected by the dark matter and develops the ability to receive visions. He is nicknamed Vibe by Barry and Caitlin.
Barry, Cisco and Harry travel together through the last open portal to Earth 2 to free Harry's daughter Jessie from Zoom's captivity. They succeed; however, they must leave behind Zooms' second prisoner, an unknown man in an Iron Mask who keeps banging on the glass of his prison cell. As they come back through the portal and are about to close it, Jay stands in front of it, and just before it closes, is killed by Zoom through it and carried away. This makes Barry realize that he needs to speed up even more in order to defeat Zoom. However, Harry and the others fear that Zoom might hurt one of their kin if they create an opening between the two dimensions again. Thanks to Cisco's visions, the team later learns that Jay Garrick is Zoom himself, and thus not dead, but has been fooling them all along. Barry sets up a speed equation to increase his speed, but can't solve it. Because of this, he travels back in time and asks Eobard Thawne for help. He gives Barry a USB mass storage device with the necessary data.
Through the data, Barry learns to move faster using tachyon energy. In the process, he briefly ends up in Supergirl's serial universe by mistake. When he returns, he is told that he has run four times faster than ever before. He decides to lure Zoom to Earth 1. Barry is able to catch him, but Zoom escapes and kidnaps Wally. Zoom wants Barry's speed in exchange, so Barry gives it up to save Wally's life. Now that Barry has no powers, Zoom sends his army to Earth 1 to conquer it.
Harry makes a plan to rebuild the particle accelerator in order to expose Barry to Dark Matter again, so that he can once again receive powers from the Speedforce. Barry agrees, but is apparently vaporized in the process. Also, Jessie and Wally are exposed to dark matter. Cisco, however, thanks to his visions, can see that Barry is still alive and trapped in the Speedforce. This, by manifesting itself in familiar people like Iris or Barry's dead mother Nora, can teach Barry to cope with his mother's death. He comes to terms with his past, regains his powers, and with the help of Cisco's new powers, returns to defeat Zoom. Full of optimism, Barry, with the help of Harry, liberates Central City from Zoom's army.
Zoom wants to show Barry that the latter is just like himself. (Zoom, real name Hunter Zolomon and on Earth 2 a convicted serial killer, had to watch as a child how his mother was killed by his father). So he kidnaps Henry Allen and kills him in Barry's childhood home, where Nora Allen was also killed by Thawne. Full of hate, Barry wants to kill Zoom, but the latter challenges Barry: he should run against Zoom, because he wants to know which of them is the fastest speedster. Team Flash, however, locks Barry up because he is in a bad mental state.
Team Flash (without Barry) tries to send Zoom back to Earth 2. This fails and Joe falls into the portal with Zoom. Now Barry wants to finish it for good and races against Zoom. The latter now decided to destroy the entire multiverse to rule Earth 1 as the fastest speedster. During the race, a second Barry arrives and takes care of the device that was supposed to destroy the multiverse. Barry beats Zoom, whereupon, as Barry had hoped, the Time Spirits suddenly come out of the Speedforce. They turn Zoom into Black Flash and take him back to the Speedforce.
After defeating Zoom, Barry explains that he traveled through time to appear twice, thereby attracting the time spirits. Flash 2 moved so fast that he vaporized while running, destroying the machine. Furthermore, it turns out who Zoom's prisoner behind the Iron Mask is: the man who, to everyone's amazement, is a doppelganger of Barry's father Henry, is the real Jay Garrick. He is Flash from Earth 3 and takes Harry and Jessie to Earth 2.
Despite the victory over Zoom, Barry, who has now lost both parents to speedsters, feels worse than ever. Iris tells Barry to do whatever he finds peace with. For the first time in this timeline, the two kiss. He then travels back in time to prevent his mother's death. He beats the Reverse Flash and tells his mother she is safe.
Season 3
After saving his mother Nora Allen in the past, Barry has drastically changed the course of time. In the beginning, however, he keeps this to himself. He keeps Reverse Flash in a cell that suppresses his Speedforce. However, as time passes, Barry loses his memories of the original timeline, and Eobard Thawne tells him that this timeline, which he christens Flashpoint, will soon overwrite the original timeline, and with it Barry's memory, until he forgets that he is Flash. To prevent this, Barry lets him go and asks him to kill his mother to restore the original timeline.
However, this has changed and has some major differences from the original timeline. For example, Iris and her father Joe have a troubled relationship. Barry therefore tries to restore the original timeline without another time jump, especially through conversations. However, the metahuman Alchemy tries to restore superpowers to the metahumans who had them in the timeline Barry created (the Flashpoint).
Although Barry has changed Earth 1 by jumping through time, Earth 2 has remained the same according to Harry. This comes to Earth 1 because his daughter Jessie has spontaneously developed speedster powers and is now Jessie Quick, which was really just a pet name of Harry's before. Harry asks Barry to stop her from using her powers, as Harry is afraid for his daughter, but later changes his mind. She is even trained by Barry for a short time so that she can successfully protect Earth 2, and is given Trajectory's old suit as a gift, now with a lightning bolt symbol on the chest. Before Harry and Jessie leave Earth 1, the team searches for one of the many other Harrison Wells from the multiverse. This one, as proof of his suitability, is to solve a riddle posed by Harry to ensure that the new Harrison Wells meets the necessary basic requirements. Harrison Wells of Earth 19, called H.R. , finally enters Earth 1 to Harry's displeasure, whereupon Harry and Jessie leave again.
With Leonard Snart and Mick Rory joining the Legends of Tomorrow, the rogue throne of Central City is now vacant and occupied by Mirror Master. However, this is beaten by Barry and Jessie as part of their training.
Caitlin Snow exhibits similar powers and physical characteristics to Killer Frost from Earth 2 in the timeline, however she initially keeps this a secret from the team and turns to her mother, who investigates her powers further and advises her not to use them for physical reasons. Eventually it turns out that H.R. is not a scientist, he could not solve the mystery, but his partner.
Caitlin then wants to find Alchemy so he can neutralize her powers. After this fails, she wears a containment bracelet to prevent an outbreak of her powers and thus her Killer Frost persona. Later, she wears a containment necklace in the shape of a snowflake given to her by the team.
Iris' brother Wally, after seeing Jessie's speedster abilities, is disappointed as he also wants to become a speedster from the bottom of his heart and didn't get any powers even though he was also exposed to the second explosion from the previous season like Jessie. Since Barry told him that Wally was Kid Flash in Flashpoint, the latter now sees only one way to make his dream come true. He seeks out Alchemy, who by now is planting painful visions in his head, and asks him to restore his powers from Flashpoint. Barry and Joe try to prevent this, but fail against Alchemy, his henchmen, and a previously unseen force. Wally is now a speedster as well and wants to help Barry as Kid Flash, but Joe forbids him. As a result, H.R. convinces Wally to train in secret. At Christmas, Wally is finally recognized as Kid Flash and is given a speedster suit.
In a flashback, Julian Albert, Barry's "new" forensics partner, is seen on an expedition to secure a mysterious artifact that he has been searching for years, believing that it will bring back his dead sister, who appeared to him in a vision. In truth, however, Savitar, another evil speedster, is trapped inside the artifact, gaining control over Julian and turning him into his puppet Alchemy. When the artifact then affects Cisco, Barry, with the help of Jay Garrick of Earth 3, throws it into the Speedforce, Barry inadvertently traveling to a possible future. There, he witnesses Iris being killed by Savitar in front of his future self, which he is unable to prevent despite his best efforts. This is a possible explanation as to why a future newspaper article in the Time Chamber of Eobard Thawne no longer shows Iris as the author. However, the team advises Barry not to affect the future and to live in the present. The removal of the artifact also causes the speedster Savitar to disappear from Earth 1. The team wonders how Savitar will return and do the deed when they threw the artifact into the Speedforce. Then it turns out Caitlin kept a shard of the artifact hoping it could help take away her powers. Barry learns that Savitar is trapped not in the artifact, but in the Speedforce itself, but that he can't free himself from there until he also gets the artifact shard. Using a ruse, Savitar gets Wally to take the artifact shard and throw it into the Speedforce, freeing Savitar again and instead sucking Wally into the Speedforce and making him relive the worst moment of his life - the death of his mother - over and over again. Along with Jay Garrick, Barry gets Wally out of the Speedforce again, with Jay staying behind at his own request to now take his place so Wally can return to Earth. Barry promises to free him.
As a result, a female so-called Collector (a meta-being named Gypsy, who has the same powers as Cisco) arrives on Earth 1 to return H.R. to his original Earth, where he is to receive the death penalty for his change of Earth according to the laws there. Cisco, however, challenges the Collector to a duel in order to protect H.R.. After winning, Gypsy admits defeat and plans to return to Earth 19 with the news that she killed H.R. to avoid admitting defeat. H.R. accepts that with the news of his death, he can't go back and thus will remain permanently on Earth 1. Barry, Iris, and the team, which has since included Julian, meanwhile try to influence the course of the future so that Iris is not murdered by Savitar. For example, Barry considers extraditing H.R., since he was seen in the future when he was murdered and accordingly has an impact on time. They also recreate the scene so that they can make arrangements, for example, Wally should help out.
Through Cisco's powers, they review the scene from the future to figure out details they need to change in order to change the entire future. This is aided by a news broadcast that is playing during the scene. Now the team sets out to stop the events that appeared in that broadcast from occurring, thus saving Iris' future. In addition, they interview a metahuman named Abra Kadabra, who claims to know how to save Iris. However, he demands his debt be forgiven in return. The team refuses and has Gypsy arrest him.
Caitlin was severely injured by shrapnel on her torso in a fight with Abra Kadabra. To avoid being revealed as a meta-being, she does not want to go to the hospital, so she asks Julian to operate on her while fully conscious under her instructions, despite his lack of medical skills. After the surgery, Caitlin's heart suddenly stops abruptly and does not resume even after several attempts at resuscitation. As a result, Julian removes the device used to suppress her abilities as Killer Frost so that her wound can heal through her meta-powers, even though she had previously specifically said that she would rather die than become Killer Frost. Caitlin transforms into Killer Frost while sending out a shockwave and gets back up.
Barry travels to the year 2024 to find out who Savitar is. He searches for the now-separated Team Flash and asks his future self about Savitar; however, he does not know Savitar's true identity. Killer Frost is imprisoned in the future and Barry asks her about Savitar. She says she won't reveal who he is, but that Barry will be very surprised. The team defeats Mirror Master and his companion Top on the side. Back in the present, Savitar reveals himself to Caitlin (Killer Frost) and they team up.
Barry has received a clue from the future. He is to find the researcher Tracy Brand, because only she can help him imprison Savitar in the Speedforce. Meanwhile, Savitar orders "Killer Frost" to kill Tracy Brand. A fight ensues between Team Flash and Killer Frost, which the team wins. Savitar then appears and takes Killer Frost away. In the evening, Joe comes home and talks about his new colleague and says that you don't know what you are and what our destiny will be. Barry then gets a flashback and thinks about everything that has been said about Savitar and what he himself has said. Barry then leaves the house and looks for Savitar. He confronts him and Barry says that he knows Savitar's identity. He then opens his suit and says, "Like I said. I'm the future, Flash." As it turns out, Savitar is a time relic of Barry himself, created by Barry to fight Savitar. Savitar kills all but one of the Time Relics in Barry's fight against him, which subsequently travels back in time to become Savitar. However, Savitar must kill Iris in order for Barry to be plunged into such a deep depression for Savitar to come into existence in the first place.
Barry informs the other Team Flash members that the true identity is Savitar's Future Barry. Cisco then develops a device that was originally intended to prevent Barry from storing new memories so that Savitar could not use them as a "weapon" against Barry. However, this plan backfires as Cisco erases all of Barry's memories due to a miscalculation. Barry as well as Savitar cannot remember their lives or identities, so Killer Frost temporarily returns to S.T.A.R Labs to reverse this. When Barry and Savitar can remember again, Tracy Brand reveals the "Speed Force Bazooka" to the team to stop Savitar. However, it requires a power source, which is located at A.R.G.U.S.
With only 24 hours left to save Iris, Barry is forced to do everything he can to prevent her death. The team hides Iris on Earth 2, but without telling Barry, to prevent Savitar from remembering her whereabouts. Lyla Michaels (Diggles' wife and director of A.R.G.U.S) doesn't trust Barry and denies him access to the power source at A.R.G.U.S. So he travels back to 1892 to bring Leonard Snart back to the present during the Legends' mission to steal the power source. This proves more difficult than previously thought, as Barry's powers are dampened in the building and the power source is guarded by King Shark. They still manage to get it, however. Meanwhile, Savitar arrives at S.T.A.R Labs undetected in his guise as 2017-Barry and asks for Iris' whereabouts. H.R is careless and reveals to him that she is on Earth 2. Savitar travels there and takes her into his custody. A little later, Barry and Savitar find themselves on Infantino Street and Cisco, meanwhile, meets with Killer Frost to talk some sense into her. However, the "Speed Force Bazooka" doesn't work properly and instead of locking Savitar in the Speed Force, she equips him with more Speed Force energy and he kills Iris.
Because of the guilt weighing on H.R. for so carelessly revealing Iris' whereabouts, he took Iris' place beforehand and changed his appearance using his light refraction technology. So Savitar unwittingly killed H.R. instead of Iris. He steals the "Speed Force Bazooka" and kidnaps Cisco so he can turn it into a "Speed Force Splicer", which will spread him across the entire timeline to rule as a god from the Big Bang to the end of the universe, and so the time paradox can't catch up with him since Iris didn't die and thus Barry won't get depressed. Hoping to bring Savitar to the "good side", Barry invites him to S.T.A.R. Labs to bring him to his senses. However, this does not work and Savitar destroys S.T.A.R. Labs with the help of the Philosopher's Stone.
Cisco completes the Bazooka rebuild and Savitar is ready to become a god and split in time. He opens a gateway to the Speed Force, however instead of being split in time, Jay Garrick is freed from the "Speed Force prison" where he had taken Wally's place. Cisco reveals to Savitar that he is responsible for the plan failing. Barry vibrates himself into Savitar's suit, destroying it in the process. Savitar lies on the ground, thought to be unconscious. Suddenly, he straightens up to kill Barry, who gloats too soon about the victory. Iris shoots Savitar and he is erased from the timeline as the Paradox caught up with him. During the fight, which also involves Killer Frost, she ultimately helps Team Flash stop Savitar. Cisco hands her the cure Julian developed for her, leaving her to choose whether to take it and become Caitlin again. At H.R.'s funeral, she meets up with Team Flash and returns the cure. She's obviously no longer the stone cold Killer Frost, but also says that Caitlin no longer exists and she has to figure out who she is now on her own.
Barry and Iris look forward to their future together with hope. But with the "Speed Force Prison" now missing an inmate who should have taken Jay's place, the Speed Force itself becomes unstable and chaos erupts throughout the city. Barry agrees to take the place in it to save the world, and after saying goodbye to everyone, goes into the Speed Force.
Season 4
Wally and Cisco now have to protect Central City without Flash. Iris is now the leader of the new Team Flash. Suddenly, a samurai robot shows up who wants to fight the real Flash instead of Kid Flash. So that's how they have to try and get Barry back. Cisco has been working on a way to get him out of the Speed Force in the 6 months since Barry disappeared. This is met with a lack of understanding from Iris, against all odds, and Cisco asks Caitlin, who can now somewhat control Killer Frost and works in a bar, to help him. An attempt to retrieve Barry apparently goes awry, but shortly thereafter a portal to the Speed Force opens in the middle of the city, from which Barry jumps out undressed. Team Flash tracks him down, but he seems to be out of his mind and is saying strange things. They then lock him up in a cell in the particle accelerator. Iris allows herself to be kidnapped by the so-called "Samuroid" in the vague hope that Barry will come to his senses and save her. Fortunately for her, the plan works. He retrieves the new suit created by Cisco and defeats the Samuroid. At the end of the season premiere, viewers are given to understand that the "Thinker" and his accomplice built the robot to bring Flash back.
Season 5
Nora reveals to Barry and Iris that she is their daughter from the future. While Barry struggles to send Nora back to her own dimension, Iris wants to develop a more intense relationship with her daughter. However, after Nora informs Barry that he will mysteriously disappear within a few years, limiting his time with his daughter, he too decides to make the most of the time he has with her.
A new enemy named Cicada with insect-like powers appears in town looking for a dangerous meta-being that steals high-tech weapons. While Barry and his team try to neutralize the meta-being, Cicada has dark intentions. Using a plan developed by Nora, Cicada is finally stopped. During the implementation, however, a member of the Flash team is in danger.
With more revealed information from Nora about Iris and Barry's future development, Iris mentally breaks down. To rebuild her, Barry asks her to help his team track the meta-being Rag Doll.
In the 100th episode, more plans are made. In order to stop Cicada, Barry and Nora want to travel back in time and gather important clues. Barry is hesitant, however, and worries about what situations his daughter might see in the past. Caitlin also gets involved in more plans to stop Cicada, bringing the entire team a whole step closer to that goal.
