"Flashback" is a science fiction anime series set in a dystopian future where humanity is on the brink of collapse due to environmental degradation and societal breakdown. The story centers around Kaito, a young man who possesses the rare ability to relive fragments of his memories through a phenomenon known as "Flashbacks." These Flashbacks are not mere recollections but vivid, immersive experiences that allow Kaito to uncover hidden truths about his past and the world around him.

Kaito's journey begins when he encounters a mysterious girl named Aria, who claims to have a connection to his lost memories. Aria is a member of a clandestine organization called the Chronos Collective, which seeks to manipulate time and memory to alter the course of history. As Kaito delves deeper into his Flashbacks, he discovers that his memories have been tampered with, and his true identity is tied to a pivotal event that could either save or doom humanity.

The narrative unfolds through a series of interconnected timelines, blending elements of mystery, action, and psychological drama. Kaito's Flashbacks reveal fragments of a catastrophic event known as the "Collapse," which wiped out much of civilization and left the surviving population fragmented and distrustful. As he pieces together his past, Kaito learns that he was once a key figure in a project aimed at reversing the Collapse, but his memories were erased to protect the project's secrets.

Throughout the series, Kaito is pursued by the Chronos Collective, who view him as a threat to their plans, and by rogue factions seeking to exploit his abilities for their own gain. Alongside Aria and a group of allies, including the resourceful hacker Rin and the enigmatic warrior Sora, Kaito navigates a treacherous landscape of shifting alliances and moral ambiguity. The group uncovers the truth behind the Collapse, revealing a conspiracy that spans decades and involves powerful entities manipulating time and memory for their own ends.

The climax of the series sees Kaito confronting the leader of the Chronos Collective, who reveals that the Collapse was not a natural disaster but a calculated act to reset humanity and create a new world order. Kaito must decide whether to use his abilities to restore the past or forge a new future, grappling with the ethical implications of altering history. The series concludes with an open-ended resolution, leaving the ultimate fate of humanity and Kaito's choices open to interpretation.