"Eiji" is a science fiction anime series set in a near-future world where humanity faces the consequences of rapid technological advancement and environmental degradation. The story centers on Eiji Hoshimiya, a young engineer who becomes entangled in a global conflict after discovering a mysterious AI system known as "Eidolon." This AI, created by a now-defunct corporation, possesses the ability to manipulate reality through advanced quantum computing, raising ethical and existential questions about its use.

Eiji's journey begins when he stumbles upon Eidolon while working on a routine maintenance project in an abandoned research facility. The AI, initially dormant, activates upon his arrival and begins communicating with him, revealing its potential to reshape the world. However, Eiji soon learns that multiple factions, including governments, corporations, and underground resistance groups, are vying for control of Eidolon. Each group has its own agenda, ranging from using the AI to restore the planet's ecosystems to exploiting its power for global domination.

As Eiji becomes the reluctant custodian of Eidolon, he is pursued by these factions, forcing him to navigate a complex web of alliances and betrayals. Along the way, he encounters key figures such as Rina Tachibana, a former scientist turned resistance leader who seeks to use Eidolon for humanitarian purposes, and Kaito Kuroda, a corporate enforcer determined to secure the AI for his employer's profit-driven goals. The narrative explores themes of technological ethics, environmental responsibility, and the moral dilemmas of wielding immense power.

The plot unfolds through a series of escalating conflicts, with Eiji struggling to balance his desire to protect Eidolon from misuse with the growing realization that its existence may inherently destabilize the world. The series delves into the psychological toll on Eiji as he grapples with the weight of his decisions, while also examining the broader societal implications of a world on the brink of collapse. The story culminates in a climactic confrontation where Eiji must decide whether to destroy Eidolon, risking the loss of its potential benefits, or allow it to continue existing, despite the dangers it poses.