The anime "Why Nobody Remembers My World?" centers on a post-apocalyptic setting where humanity is on the brink of extinction due to the collapse of civilization. The story unfolds in a world where advanced technology and magic once coexisted, but a catastrophic event known as the "Great Collapse" erased most of humanity's knowledge and history. The remnants of society are divided into factions, each struggling to survive in a harsh, desolate landscape.

The protagonist, Kaito, is a young man who possesses a unique ability: he can remember the world as it was before the Great Collapse. This memory sets him apart from others, as most people have forgotten the past entirely. Kaito's recollections of advanced technology and lost civilizations make him a target for various groups seeking to exploit his knowledge. Among these groups is the powerful organization known as the "Eclipse," which aims to control the remnants of the old world's technology to dominate the new one.

Kaito is joined by a group of allies, each with their own motivations and abilities. Among them is Sora, a skilled fighter with a mysterious past, and Rin, a young girl who possesses latent magical powers. Together, they navigate the dangers of the post-apocalyptic world, facing off against rival factions, mutated creatures, and the remnants of ancient technology that still lurk in the ruins of the old world.

As the story progresses, Kaito and his companions uncover the truth behind the Great Collapse and the role that the Eclipse played in the downfall of civilization. They learn that the organization's leaders are attempting to recreate the old world's technology to establish a new order, one that would subjugate the surviving population. Kaito's memories become crucial in thwarting their plans, as they contain the key to unlocking the secrets of the past and preventing history from repeating itself.

The narrative explores themes of memory, identity, and the consequences of technological advancement. Kaito's struggle to reconcile his memories of the old world with the harsh realities of the present drives much of the story's tension. The series also delves into the moral ambiguities of survival in a world where resources are scarce and trust is a rare commodity.

Throughout the series, the characters face numerous challenges, both external and internal. Kaito's journey is one of self-discovery, as he grapples with the weight of his memories and the responsibility they entail. The relationships between the characters evolve as they confront their own fears and desires, adding depth to the overarching plot.

In the final arc, Kaito and his allies confront the leaders of the Eclipse in a climactic battle that determines the fate of the world. The resolution of the story hinges on Kaito's ability to use his memories to outwit his enemies and restore a semblance of hope to the shattered world. The series concludes with a bittersweet ending, leaving room for interpretation about the future of humanity and the legacy of the old world.