While visiting his mother's grave, Barry Allen, known as the Flash, is alerted to a break-in by Captain Cold, Captain Boomerang, Heat Wave, Mirror Master, and Top at the Flash Museum. Defeating The Rogues, he discovers that they have been hired by his archenemy Eobard Thawne/Professor Zoom as part of a plan to destroy Central City. With the help of the Justice League, Flash foils Zoom's plot, but Zoom's taunts over the death of his mother still haunt Barry as he departs.
The next day, Barry discovers that the world has radically changed: his powers are gone, his mother is alive, his wife Iris is married to someone else, and the Justice League does not exist. Aquaman and his Atlantean forces have sunk most of Europe, while Wonder Woman has led the Amazons in conquering Great Britain, and both forces are now at war after an attempted alliance between them led to an affair between Aquaman and Wonder Woman, and the latter murdering Aquaman's wife Mera after a confrontation. Cyborg has assembled a team to eliminate both parties and approaches Batman to join them, but his refusal leads the government to scrap the plan, and instead recruit Hal Jordan to pilot an extraterrestrial spacecraft to bomb the Atlanteans.
Barry visits Wayne Manor, but is attacked by BatmanTemplate:Mdashwho he realizes is not Bruce Wayne but his father Thomas. Attempting to explain things to the elder Wayne, Barry retrieves his costume from his ring, but Zoom's costume appears, causing Barry to believe Zoom is responsible for the timeline alteration. Barry convinces Batman to help, and Batman helps him recreate the accident that gave him his powers, but Barry is severely burned.
In London, Steve Trevor attempts to extricate reporter Lois Lane, but is discovered by the Amazons and killed. The Amazons hunt down Lane, but the local resistance rescues her. Meanwhile, in the remains of Paris, Slade Wilson/Deathstroke and Lex Luthor are attacked and killed by Aquaman's forces while tracking the energy trail of Aquaman's new weapon, which is powered by the captive Captain Atom.
At the Batcave, after realizing that his memories are changing, Flash asks Batman to recreate the accident again. The second attempt results in success and Flash's powers are restored, but he discovers he cannot travel through time because Zoom is also using the Speed Force. Flash recruits more allies, beginning with Superman, who is a prisoner of the U.S. Government after his ship crashed into Metropolis. With the aid of Batman and Cyborg, they liberate Kal-El, originally known as Clark Kent/Superman who is empowered by Earth's yellow sun for the first time and fends off the facility's security then flies off in fear and confusion. Flash collapses as his memories continue to change.
Flash is taken to Billy Batson's home to recuperate, and learns that Hal's attack has failed and that the final battle between the Amazons and Atlanteans has begun. Flash convinces the superheroes to help stop the war and they depart for Britain aboard Batman's jet, only to be shot down upon arrival. Billy and his siblings combine into Captain Thunder to fight Wonder Woman, while Flash, Cyborg and Batman occupy Aquaman. Batman is wounded as Zoom reveals himself and explains that Flash is responsible for this alternate timeline: Barry traveled back in time to save his mother, fracturing the fabric of reality. Wonder Woman uses her lasso to force Captain Thunder back into Billy and kills him; Kal-El arrives but cannot save Cyborg from Aquaman, who then remotely detonates his Captain Atom-powered bomb. As the explosion tears across the landscape, Zoom mocks Flash as he attempts to run back in time but is killed by Batman. The dying Thomas urges Flash to run, giving him a letter addressed to his son as he dies from blood loss. Flash races back in time and stops his earlier self but again fractures time, creating another alternate timeline.
Barry awakens at his desk and finds that reality is apparently back to normal. After spending a moment with Iris at his mother's grave, Barry visits Bruce Wayne to tell him about everything that occurred. Flash gives his Batman the letter from his father, and Bruce thanks Flash, who then runs off. In a post-credits scene, a Boom Tube opens in space above Earth and a horde of Parademons emerges.
- Justin Chambers as Barry Allen / The Flash
- Kevin McKidd as Thomas Wayne / Batman
- C. Thomas Howell as Eobard Thawne / Professor Zoom / Reverse-Flash
- Michael B. Jordan as Victor Stone / Cyborg
- Nathan Fillion as Hal Jordan / Green Lantern
- Ron Perlman as Slade Wilson / Deathstroke
- Kevin Conroy as Bruce Wayne / Batman
- Dana Delany as Lois Lane
- Cary Elwes as Orin / Arthur Curry / Aquaman
- Vanessa Marshall as Princess Diana / Wonder Woman
- Dee Bradley Baker as Etrigan, Top, Canterbury Cricket
- Steve Blum as Lex Luthor, Captain Thunder
- Sam Daly as Clark Kent / Kal-El / Superman
- Grey DeLisle-Griffin as Nora Allen, Young Barry Allen, Martha Wayne
- Jennifer Hale as Iris Allen, Billy Batson
- Danny Huston as General Sam Lane
- Danny Jacobs as Grifter, Captain Cold
- Peter Jessop as Dr. Vulko
- Lex Lang as Captain Atom, Funeral Presider
- Candi Milo as Persephone, Pedro Pena
- Kevin Michael Richardson as President of the United States, James
- Andrea Romano as Doris, Central City Newsreader
- James Patrick Stuart as Steve Trevor, Ocean Master, Captain Boomerang
- Hynden Walch as Yo-Yo
|Animated DC Films|