"Daikyū Shintai" is a manga series that explores themes of identity, transformation, and the human condition through a blend of psychological and supernatural elements. The narrative centers around a young man named Shintaro, who discovers that his body is not entirely his own. After a near-fatal accident, Shintaro awakens to find himself fused with a mysterious, sentient entity known as Daikyū. This entity grants him extraordinary abilities, including rapid regeneration and enhanced physical capabilities, but at a cost: Daikyū has its own consciousness and agenda, often conflicting with Shintaro’s desires and morality.

As Shintaro struggles to coexist with Daikyū, he becomes entangled in a series of events involving other individuals who have undergone similar transformations. These individuals, referred to as "Hosts," are bound to entities like Daikyū, each with unique powers and personalities. The Hosts are drawn together by an enigmatic organization that seeks to control or eliminate them, believing their existence to be a threat to humanity. Shintaro must navigate this dangerous world, balancing his own humanity with the growing influence of Daikyū, while uncovering the truth behind the entities and their origins.

The story delves into the psychological toll of sharing a body with another being, as Shintaro grapples with questions of autonomy, free will, and the nature of self. His relationship with Daikyū evolves from one of antagonism to a fragile partnership, as they face external threats and internal conflicts. Supporting characters, including other Hosts and members of the organization, add depth to the narrative, each with their own motivations and struggles. The manga explores themes of coexistence, sacrifice, and the blurred line between human and non-human, culminating in a tense and thought-provoking exploration of identity and survival.