"AFTERLOST" is a post-apocalyptic fantasy series set in a world devastated by a catastrophic event known as the "Collapse," which obliterated much of civilization. The story centers on Arata, a young man who awakens in the ruins of Tokyo with no memory of his past. He discovers that the city is now overrun by mysterious, monstrous creatures called "Shinjuu," which are drawn to a rare and powerful energy source known as "Shinjuu Cores." These cores are remnants of the Collapse and hold immense power, making them highly sought after by survivors and dangerous factions.

Arata encounters a girl named Yuki, who possesses a unique ability to manipulate Shinjuu Cores. Yuki is determined to uncover the truth behind the Collapse and the origins of the Shinjuu. Together, Arata and Yuki form an uneasy alliance, traveling through the desolate landscape in search of answers. Along the way, they encounter other survivors, each with their own motivations and secrets, including a group known as the "Seekers," who are hunting Shinjuu Cores for their own purposes.

As the story progresses, Arata begins to recover fragments of his memory, revealing a connection to the Collapse and the Shinjuu. Yuki's abilities also become a focal point, as her power to control Shinjuu Cores raises questions about her true nature and the potential consequences of her actions. The narrative explores themes of survival, identity, and the ethical dilemmas surrounding the use of power in a fractured world.

The series balances action and mystery, with the characters' journey intertwined with the larger mystery of the Collapse and the Shinjuu. The dynamic between Arata and Yuki evolves as they confront external threats and internal conflicts, driving the plot forward. The world-building emphasizes the bleakness of the post-apocalyptic setting while introducing elements of fantasy and science fiction, creating a unique backdrop for the unfolding story.