After preventing the robbery, Jay Garrick (the original Flash) and Wally West (the third Flash) enjoy the Christmas Eve with Joan and Linda in Jay's house. And then, someone knocks on the door, everybody is shocked to see Barry Allen, the Flash before Wally West and his uncle and mentor who died, alive and well.
A week later, Jay and Hal Jordan (the Green Lantern) are happy to see Barry alive who according to Barry himself, reformed from a being of energy into a human during Wally's fight with Doctor Alchemy. He could not remember who he is until he visited Flash Museum and stole one of the older Flash costumes he found there and the next thing he did was found Jay and Wally. While Wally is very skeptical about Barry's return, according to Hal's ring, Barry is telling the truth and Hal tells Wally that he should not worry. He knew Barry for a long time as they were best friends. And then, Hal takes of.
As they walk out on the streets, Barry sees his old enemy, Pied Piper and attacks him with full force, extremely aggressive and unusual behavior for Barry. As Wally tells him that Piper is his friend now, Barry is skeptical because of the fact that Wally revealed his secret identity to the public but before the two could end the discussion, Piper informs Wally that there is a gang war going on and it is about to turn the city upside down. As Wally suits up, Barry compliments his new costume while Wally was just asking himself what will Barry's reaction be as he never told him he is the new Flash now.
Just as they stopped the assassination of Jack Giacamo, Barry tells Wally to avoid for him and he needs to take care of it. Wally followed him and he saw Barry visiting the grave of his wife Iris. He remembers how happy they were, especially when she accepted his proposal and tells her he loves her. Barry thinks about how proud she would be if she saw Wally in action and he knows that Wally does not believe him. However, Wally heard everything Barry said and tells him he is sorry for not believe he came back. His death hurt him pretty badly and he wasn't happy when he saw him, he was afraid. Tomorrow starts a new year and people have to move on with their lives. And just like that, the two speedsters run together.
Few weeks later, Wally and Jay stop a gang of high tech equipped commanders. Unfortunately, the train ran out of control and was about to crush civilians. Jay and Wally were unable to stop it but with Barry's help, they were successful. Civilians ask the Flash for autographs but just as Wally was about to give them, Barry is the one who wrote the autographs. Back home, he talks to Linda about taking a different name considering the fact that Barry came back and wants to move out of the city which Linda does not really agree with. As the two laugh over different ideas for Wally's potential new alias, Jay calls, telling them that the commanders they fought on the train took over a warehouse but before he could go there, Wally saw Barry standing in the same alley he came back from. He feels weird considering the fact that he was dead for years but to him, it all seemed like seconds. The two speedsters start walking but it seems that there is a book next to a trash container. During the fight, the commanders shot laser at Wally that tore the ground apart, slamming pieces of rock on to Wally. The commander screams he killed the Flash and at the moment he heard it, Barry asked him what he said and blitzed the commander with rage in his eyes, beating him up. Wally stopped Barry from beating the commander to death. Wally believed Barry acted that way because he thought the commander killed Wally but what Barry actually said that nobody else heard is Barry screaming how he is the Flash. He almost killed the man for calling someone the Flash other than himself.
Wally asks himself what is happening to Barry as he never acted the way he does now. He reads a message from Pied Piper about the new location of the commanders. Meanwhile Jay Garrick visits his own friend from World War II, one of the original speedsters Johnny Champers, or as he is commonly known, Johnny Quick. He asks him for help but they both agree they will need Max Mercury for this problem as well. They meet Max who currently sells metro tokens. Max for reasons unknown claims that he is not Max Mercury and they got the wrong guy. Jay drops his tokens, making Max reveal his reflexes and reminds Max that he owns him one. As he agrees, Jay gives Max his old costume which he kept all those years because he wanted to remember him. While Wally was reaching his location, Barry met him and they team up. However they were trapped in a force field they can't penetrate or phase throw. Wally finds a way to save Barry and vibrates on the force field to allow him to phase throw and as he asks Barry to help him. Barry refuses, telling him he stole his name and identity, trying to make Barry Allen a forgotten history but he will make everyone remember who Barry Allen was and still is, by destroying Central City. Wally begs Barry to understand he never wanted to replace him, he wanted to honor his memory but Barry does not listen and leaves him to die. Wally only focuses on vibration and phases through the force field. He comes back home and sees Barry on TV having a Press Conference where he said that his protege Wally West, former Kid Flash is not dead. Hearing it, Wally throws his costume in the trash, feeling nothing but grief.
Barry started to terrorize Central City which drew the attention of Darkstar who tried to kill him but was stopped by Hal Jordan. However, Hal chased Barry to the Flash Museum, asking him why he is doing all of this but Barry only wants to make them remember him and beats up Hal. While Darkstar tried to kill him again, Barry got away.
Next day, Barry attacked construction workers at construction site but they were all saved by Jay Garrick, Johnny Quick and Max Mercury. Despite their combined efforts, they were unable to stop Barry who left construction site once the news helicopter was about the crash but the reporters were saved by Jay and Max. Meanwhile Wally came to the alley where Barry appeared when he was resurrected and he found a book about the Flash from a homeless person nearby which gave him the answer about all this crisis.
Johnny, Max and Jay meet Wally, talking about his speed. Johnny mentions the formula he invented which gave him super speed. Max is trying to train Wally and teach him how to get faster and tells Wally how he is afraid of replacing Barry which limits his powers. Jay on the other hand tells Wally he deserved the name the Flash and from now on he will call him that way and will no longer use the word junior. After Wally showed them the book he found, he had an idea how to stop Barry and he went to the Flash Museum. Jay, Johnny and Max were searching for Barry who freed all of the prisoners from their cells and starting a riot which was easily stopped by Johnny and Max but Barry defeated Jay in a fight by breaking his leg and he knocked down Max and Johnny. He ran back to the Flash Museum, just the place where Wally waited for him to show up. Wally gave him the ring with the suit inside which looked familiar. Wally find out everything. Somehow he did not full them but himself which made Hal's ring think he was saying the truth. He was confused because he dropped the book which was revealed to be The Life Story of The Flash, Barry's biography from life to death. It could not have been written in this time because it is from the future. As Wally tells the man who everyone thought was Barry Allen all this time all this, the man puts on a suit and it was revealed that all this time, the man who claimed to be Barry was actually Professor Zoom, the Reverse Flash, Eobard Thawne, Barry's arch enemy from the 25th Century.
Two speedsters, one good and one evil start to fight at the remains of the Flash Museum, Wally tells Professor Zoom how Barry killed him years ago by snapping his neck in order to stop him from killing his fiance. Thawne explains how he still did not meet Barry Allen. He came from the timeline where technology was so advanced the crime did not exist but it was also the era without super heroes. However, there were people who felt nostalgia for super heroes. Thawne was one of them. He adored Barry Allen. He was worshipping him, he was his hero. Thawne was always reading about him, he knew everything about Barry and the only thing he wanted is to meet his hero. He changed his face to make himself look like Barry and wanted to use the Cosmic Treadmill to travel back in time and meet his hero before he died by replicating the accident that gave Barry his super speed. However, Thawne traveled into the wrong era, years after Barry's death. Thawne visited the Flash Museum, realizing he was brought to this timeline for a reason, to continue the legacy of Barry Allen and become the new Flash of this timeline. Because of his new goal, he begin to study various enemies of the Flash, especially the Reverse Flash, his greatest enemy. Because the Reverse Flash's name was unknown in this timeline and in the 25th Century, Thawne did not know that he was Reverse Flash and when he found out his future identity plus the fact that his greatest hero will be his murderer, the shock put Thawne in the state of extreme confusion and madness, making him believe that he was Barry Allen.
Wally plans on stopping Thawne but he has no chance as he is much faster than Wally. Thawne will not allow himself to be removed from this timeline as he is away from Barry and the events of his death so he tries to destroy the cosmic treadmill. But because the treadmill is the item that gave Thawne access to his timeline which is the only thing preventing him from dying at the hands of Barry Allen, Wally tells him not to destroy the treadmill or he will reset the timeline. Because of that, Thawne refuses to destroy the treadmill and escapes after Wally was saved by Max, Johnny and the Green Lantern. In the hospital where Jay is right now, Wally plans to send Thawne back to his timeline where he will eventually become Reverse Flash once again. Max once again reminds him how he needs to stop being afraid of replacing Barry by becoming his equal. On the news, Thawne admits he is the one who was terrorizing the city, not the Flash who is indeed dead. As he was about to kill Linda, he was stopped by Thawne who easily defeated Wally once again. When he finally admitted he was afraid of replacing Barry, Wally realizes his speed never left him, it was just limited by his own psychic blocks and not they returned. Wally overpowers Thawne who begs for mercy, but Wally only mocks him, calling him pathetic and a victim of his own obsession. Wally jumps on the cosmic treadmill, telling Thawne how he might travel into the future and terrorize him like he terrorized him, or maybe even kill him. As Thawne uses the treadmill to travel into the future to stop Wally, it turns out that Wally just ran so fast it looked like he disappeared. He wanted Thawne to travel into the future in order to set the timeline the way it was suppose to be.
The Flash Museum was repaired after getting destroyed in Thawne's fight with Hal but now they added the statue of Wally West next to his mentor Barry. Wally finally gets out of his uncle's shadow but he knows he will always be there watching for him. Wally looks at Barry's biography which turned out to be written by his Aunt Iris, Barry's wife and she is about to write it only few years from now, meaning that she was alive and not killed by Thawne. Wally throws the book into the river because he can't find out what the history has in store for him, because his history belongs only to him.