Barry Allen, also known as the Flash, wakes up in an alternate reality without his powers. Confused and disoriented, he soon discovers that he no longer possesses his speed abilities. As Barry tries to understand what has happened, he realises that this alternate reality is vastly different from the one he knows.
Barry seeks help from his fellow superhero, Batman, and finds him in this alternate timeline leading a darker, grittier life. Batman is more brutal and violent in this reality, reflecting the grimness of the world around him. Batman agrees to help Barry but warns him about the risky nature of tampering with time.
Barry realises that he must travel back in time to prevent whatever event caused this alteration. With Batman's guidance, he navigates through the time stream, but their efforts are interrupted by an army of Atlanteans and Amazons at war. The two heroes eventually consult with the renowned scientist Cyborg, who is now a government agent, and discover that the war between the Atlanteans and Amazons is tearing the world apart.
Barry realises that the key to restoring the timeline lies with the superhero team, the Justice League, who have never come together in this altered reality. He seeks out familiar faces, including Wonder Woman, Green Lantern, and Aquaman, only to discover they are vastly different. Wonder Woman is no longer a hero but a fierce warrior queen engaged in a romantic relationship with Aquaman, who has conquered Europe in this reality. Green Lantern, on the other hand, is merely a pilot, lacking the superhero alter ego.
While Barry tries to convince the altered superheroes of their true selves, Batman investigates the cause of this altered reality. He discovers that Professor Zoom, a villainous speedster, is responsible for altering the timeline. Batman manages to capture Zoom but is killed in the process, leaving Barry devastated.
Barry decides to use Zoom's time-traveling device to prevent Zoom from ever altering time. However, as he saves his mother from being murdered, he unintentionally creates an even worse reality. The world descends into chaos as the Atlanteans and Amazons wage an all-out war against each other, and metahumans are imprisoned and experimented on.
Barry, realizing his mistake, desperately seeks out the help of a now-aging Batman, who is Thomas Wayne (Bruce Wayne's father). Thomas reluctantly agrees to help Barry reverse the damage in this new reality. They gather allies, including a resistance group led by Cyborg, and plan to infiltrate the Batcave, which is now occupied by the villainous Reverse-Flash.
During the intense battle in the Batcave, the heroes face numerous challenges. Barry confronts Reverse-Flash, who reveals that he orchestrated these events to hurt Barry emotionally. Barry defeats Reverse-Flash and prepares to undo the alterations using the time-traveling device.
However, as the timeline begins to reset, Barry is forced to say his goodbyes. He visits his mother and bids her farewell, knowing that her survival is not meant to be. Barry, filled with remorse, races back to the present, where he finds the world restored to a version similar to his original timeline.
Though relieved to be back, Barry is haunted by the sacrifices made and the realization of the dangers of tampering with time. The movie ends with Barry attending a crime being committed, ready to resume his role as the Flash, but with a burden of guilt and a newfound appreciation for the fragile nature of reality.
Answer: Flash couldn't travel through time later in the movie not because Zoom was alive but because Zoom was also tapping into the Speed Force, preventing Flash from gathering enough of the Speed Force to break the Time Barrier. When Flash originally went back in time it's likely because Zoom was in custody, therefore not tapping into the Speed Force at that time.