After an unexpected guest from the future, Nora West-Allen, appears at their home, Barry and Iris must figure out how to get her back to the future without disrupting the timeline, even more than she already has. Team Flash must work together to send Nora back, while simultaneously fighting off another villainous meta.

Earth-2 Wells is captured while running through the woods. Jesse explains that he was leading an expedition to Gorilla City that was ambushed and killed while Wells disappeared. Barry recalls that in the future Central City will be attacked by gorillas. He, Julian, Cisco, and Caitlin go to rescue him but are captured by Grodd. Speaking through Wells, Grodd asks Barry to help him usurp his master Solovar. Barry agrees to fight Solovar in the arena for the lives of the others and wins, but refuses to kill him. Grodd seizes control of Gorilla City and prepares an invasion of Earth-1. The team asks Caitlin to kill Cisco so Grodd can't open a portal, but she refuses. Barry fakes his death and Grodd removes him from his cage. He resuscitates himself and frees the others, and they return to Earth-1. Jesse and her father reunite and Wally and Jesse recommit to their relationship. Julian asks Caitlin out on a date. Grodd assembles his army with a brainwashed Gypsy at his side.

Barry and Wally continue their training in order to save Iris from Savitar. They race against each other, and Barry wins by phasing through a building. Julian finds a decayed corpse that was murdered just eight hours ago. He informs Joe that this is surely the work of a metahuman. The body is taken to S.T.A.R. Labs for investigation, where Caitlin and Julian observe it turning to ash. Iris confesses to Barry that she is afraid of her future. The meta responsible for the disintegrated corpse attacks Joe and is revealed to be targeting the police who arrested him in Flashpoint. The metahuman tries to disintegrate a bridge on the train Joe is on, forcing Barry to phase the train through the wreckage of the bridge. Barry teaches Wally how to phase, which Wally uses to expose the meta to his blood and strip him of his powers. Joe insists that everyone be honest with him from now on after learning of Iris's future death. While practicing phasing, Wally is shocked to see Jesse coming through a portal. Jesse says her father has been abducted by Grodd and taken toGorilla City.

An Earth-19 bounty hunter named Gypsy, possessing powers similar to Cisco's, arrives on Earth-1 intending to take H.R. back to Earth-19 for trial and execution, as inter-dimensional travel is forbidden there. Cisco demands a trial by combat for H.R.'s life, which she accepts. As the duel is to the death, Barry and H.R. try to ambush Gypsy to save Cisco, but she easily defeats them. Meanwhile, determined to leave a legacy, Iris persuades Wally to help her bust an arms dealer. While he is out scouting, she confronts the dealer, seemingly unafraid of death.

Barry has been having nightmares about Iris's murder. Jared Morillo attempts a robbery, but is confronted by Barry, who recalls a news report during his visit in the future when the latter captures the former. The distraction allows Morillo to overpower Barry and escape. H.R. launches the museum, but no one visits it. In a second confrontation, Wally intervenes and captures Morillo, gaining praise by everyone except Barry. He eventually reveals the future he saw to everyone but Joe. The team decides to change the course of other events in order to prevent Iris's death. Using Cisco, Barry accesses the future memory, in which the news reveal that the S.T.A.R. Labs museum will close and Caitlin will become Killer Frost. Morillo escapes, and Barry decides to change the first event by letting Wally capture him. Cisco relaunches the museum, starting with an exhibition for elementary school students. Julian is persuaded by Caitlin to join the team. A mysterious woman from another Earth arrives, searching for H.R.

Barry asks Jay Garrick for help against Savitar. The two heroes locate and defeat Alchemy and return his weapon to its box, causing Savitar to disappear before it can kill Jay. Discovering Julian to be Alchemy, Barry reveals his own identity to him to get further information. Julian relates how he located the Philosopher's Stone following a vision of his dead sister, and that he has been having blackouts ever since. Savitar manifests through Cisco using visions of Dante and fights Barry and Wally, but Caitlin manages to convince Cisco to close the box again. The team communicates with Savitar through Julian, who claims that Barry, in the future, will imprison him. He also says that one teammate will betray them, another will "fall", and a third will suffer a fate worse than death. Trying to end Savitar's threat, Barry and Jay throw the box into the Speed Force, but in the process, Barry is thrown five months into the future and witnesses Savitar killing Iris. Jay advises Barry that the future is not fixed. Wally is given his own "Kid Flash" uniform, Julian gets Barry's job back for him, and Barry rents a new apartment for him and Iris.

Barry investigates a meteor crash outside Central City that turns out to be a spaceship from which aliens emerge. Lyla tells the team that the "Dominators" landed previously in the 1950s, but then mysteriously departed. Needing help, Barry assembles the original Team Arrow, Thea, the Legends, and Supergirl of Earth-38. The team begins training at a S.T.A.R. Labs facility, sparring against Supergirl to prepare against the aliens. Cisco finds and reveals a message to Rip Hunter from Barry's future self, which exposes Barry's manipulation of the timeline and how it affected other team members. Oliver, Supergirl, Felicity, Martin and Jefferson become the only ones who still trust Barry. The Dominators abduct the President. Supergirl leads most of the others in a rescue effort, but the Dominators kill the President and trap them with a mind control device. The controlled heroes return and attack S.T.A.R. Labs. While Oliver holds them off, Barry lures Supergirl to the device and manipulates her into destroying it, freeing everyone. The team decides to trust Barry again. Suddenly, Sara, Ray, Diggle, Thea, and Oliver are teleported away before Barry can intervene.

This episode begins a crossover that continues on Arrow season 5 episode 8 and concludes on Legends of Tomorrow season 2 episode 7.

Savitar opts instead to race Barry across the city, then batters him to demonstrate its superiority. Cisco and Caitlin arrive and rescue Barry, but Savitar flees. Caitlin, with a Killer Frost aspect having taken over, violently interrogates one of Alchemy's followers at CCPD headquarters, then abducts Julian and coerces him to find other acolytes, in order to find Alchemy and have him remove her powers. Barry and Cisco track her down and imprison her, but the team is fractured after she reveals Barry's role in Dante's death by creating "Flashpoint". Greatly concerned and disregarding the consequences, Joe breaks Wally out of the cocoon. Wally emerges with unmanageable speed powers and he races away in a confused state. Barry offers his life to Caitlin for her freedom in a successful attempt to restore her sanity and gain her help in treating Wally. Joe and Barry locate Wally and stabilize his condition with Caitlin's serum, and Wally soon embraces his new abilities. Julian forces Barry to resign from the CCPD in exchange for him protecting Caitlin from the police. Julian is later revealed to be unwillingly working for Savitar as Alchemy.

When Wally relates his recent dreams of being Kid Flash, Barry reveals what happened to him during "Flashpoint". A metahuman H.R. nicknames "Shade" appears and murders a stockbroker. Caitlin reveals her powers to Cisco and has him "vibe" her to know her future. Cisco sees himself fighting Killer Frost, causing Caitlin to contemplate leaving for good. Alchemy begins summoning Wally to him, so the team locks him in the particle cells for his own safety. Meanwhile, Barry and the others stop Shade's next attack and capture him using meta-dampening handcuffs. Cisco tells everyone the truth about Caitlin, and Barry privately confesses to her that her powers came from him creating "Flashpoint". Wally volunteers to lead the team to Alchemy to capture the villain and stop the memories, which are physically injuring him. Barry, Joe and a S.W.A.T. team locate Alchemy and his followers and surround them. Another speedster, which only Barry can see, suddenly attacks them. As Barry fights it, Alchemy entices Wally to pick up his fallen energy weapon, which encases Wally in a crystal cocoon. Calling itself "Savitar", the speedster overpowers Barry and prepares to stab him.

Without informing the others, Caitlin visits her estranged mother, Carla, at her medical research company to have her powers examined, but leaves after viciously attacking another scientist who intended to exploit her condition. Meanwhile, a gigantic creature repeatedly appears and disappears on the streets of Central City. Cisco grows suspicious of the newly-recruited "H.R." Wells when he does not provide much help in battling the monster. The monster is revealed to be simply a hologram operated by a bullied 15-year-old boy wanting to create fear in others. Barry exposes the deception in time to keep police snipers from harming civilians, and prevent Julian from shooting the boy, whom he mistakes for a metahuman. H.R. is revealed to have no technical knowledge at all. He admits that he is only an "idea man" for his tech company on Earth-19, and that he came to Earth-1 primarily to get material for a novel. The team decides to give him a chance to prove his worth as an assistant. A badly shaken Julian reconciles his differences with Barry. Carla informs Caitlin that her powers are increasing and that she needs to stop using them, or her condition will become irreversible.