After aiding Bruce Wayne as Batman and Diana Prince as Wonder Woman in foiling a robbery in Gotham City, Barry Allen returns to his childhood home. There, he reminisces about his parents, Nora Allen and Henry Allen, before Henry's unjust conviction for the murder of Nora. On the fateful day of her death, Nora had sent Henry out for a forgotten can of tomatoes, leaving her defenseless in the kitchen, which ultimately led to her murder by an unknown assailant.
Overcome with emotion, Barry unintentionally taps into the Speed Force, crafting a "Chronobowl" that enables him to journey back in time to that very day. Ignoring Bruce's cautionary words about the unforeseen repercussions of time travel, Barry travels back and discreetly places the can of tomatoes into Nora's grocery cart, thinking this simple act would keep his father home and safe. However, as he returns to the present, he is forcefully ejected from the Chronobowl by a mysterious speedster and finds himself in a 2013 where Nora is still alive. Here, he reunites with his parents and his younger self, realizing this is the pivotal moment when he first acquired his extraordinary powers.
To ensure his past self attains superpowers, Barry and his younger version head to the Central City Police Department, where Barry reenacts the moment that led to the fateful lightning strike for 2013-Barry. Both of them end up getting struck by lightning, resulting in 2013-Barry gaining superpowers while Barry himself loses his abilities. As Barry endeavors to train his younger counterpart in mastering his new skills, they soon discover that General Zod is planning an invasion of Earth.
In a desperate bid to combat Zod, the Barrys set out to assemble the Justice League but quickly find themselves at a dead end. In this altered timeline, Diana is nowhere to be found, Victor Stone has yet to acquire his powers, and Arthur Curry has never been born. Determined, they travel to Wayne Manor in search of Bruce, only to be met by an alternate version of him who has long hung up his cape. Bruce postulates that their tampering with time has consequences that ripple throughout both past and future events. Convincing Bruce to return as Batman, they work together to locate Kal-El / Superman.
After a search, they stumble upon a Kryptonian pod in Siberia, but instead of Kal-El, they find his cousin, Kara Zor-El, held captive. They manage to free Kara and take her back to Wayne Manor. Barry seeks Bruce's assistance to reclaim his powers, but their initial attempts are fruitless. In a bold maneuver, Kara lifts Barry into a storm, allowing him to regain his abilities. With their powers restored, Kara and Batman join forces with the Barrys to confront Zod and his army.
During the intense battle, Kara discovers that Zod had intercepted Kal-El's escape pod and killed him while attempting to extract the vital Codex needed for the resurgence of their race. In a devastating twist, Zod reveals that the Codex is contained within Kara herself. Tragically, Zod defeats Kara, extracting the Codex from her blood, as Batman heroically sacrifices himself in a futile bid to destroy Zod's spacecraft.
Frustrated by their repeated failures to save Batman and Kara, the Barrys continuously travel back in time. Barry realizes that certain events in life are immutable, yet 2013-Barry remains resolute, insisting on trying to alter the timeline. As 2013-Barry makes another attempt, the multiverse begins to collapse. The mysterious speedster reappears and is revealed to be an older version of 2013-Barry, who is fixated on defeating Zod and salvaging his world. When Barry expresses his resolve to revert the changes he made and allows Nora to die, the bitter elder 2013-Barry tries to kill him but ends up mortally wounding his younger self. In a selfless act, young 2013-Barry sacrifices his life to save Barry from his future counterpart.
With the timeline restored, Barry comes to terms with his mother’s death but makes a minor alteration by placing the tomato cans on a higher shelf. Upon returning to the present, Barry attends Henry's court hearing, where new security footage reveals Henry reaching up to grab the tomato can, leading to his exoneration. Ultimately, Barry receives an unexpected call from Bruce, whose appearance has transformed once more due to the timeline changes.
