Junichi Naruse is a 22-year-old ordinary engineer who, while searching for an apartment with his girlfriend Kei, accidentally becomes a victim of a robbery at a real estate agency. In an attempt to save a child from the robber, Junichi is shot in the head. Miraculously, he survives thanks to a cutting-edge medical procedure that involves a brain transplant.

However, Junichi soon realizes that he's not alone in his own head—there's another presence. As he recovers his memory, he discovers two brains encased in glass labeled 'Host JN' and 'Donor No. 2.' The second brain is the remainder of the donor's brain used to replace the parts destroyed by the gunshot.

Junichi's life seems to return to normal, with happy days with Kei, but small changes begin to occur within him, leading to growing discomfort and the mystery of the donor's identity.

This manga adaptation of Keigo Higashino's novel 'Metamorphosis' is a gripping mystery that explores the implications of a brain transplant and the quest for one's true self.