A sudden zombie outbreak ravages Japan, and high school student Yuuma Kousaka is bitten while trying to escape. Convinced he is about to die, he sits by a riverbank waiting for the virus to take over. There he reunites with Haruka Hyuuga, the younger sister of an old friend. Taking pity on him, she agrees to fulfill his dying wish by becoming his girlfriend and lets him touch her breasts as a final, irreplaceable memory.

But Yuuma does not die. Overwhelmed by his desire to protect Haruka, he miraculously retains his consciousness through the transformation. He soon discovers he can now switch between his human and zombie forms at will. With newfound zombie abilities, he becomes the strongest person in this apocalyptic world. However, he quickly learns that physical strength means nothing against Haruka, who knows all his weaknesses and now holds their relationship agreement over his head.

As Yuuma and Haruka navigate this new reality, they encounter other survivors, including a girl named Mai Tsukishiro. Yuuma makes a startling discovery his saliva can halt or reverse zombification. This forces him to periodically kiss Mai to prevent her from fully turning into a zombie. The situation grows even more complicated when they meet a pair of college-aged survivors. To hide Mai's condition, Yuuma and Mai are forced to pretend to be a couple. This charade does not amuse Haruka, who considers herself Yuuma's real girlfriend despite his assumption that their agreement ended when he survived.

As the group struggles to survive, Yuuma finds himself caught between genuine zombie horror, the demands of keeping infected companions alive through physical contact, and the romantic chaos of a girl who believes he owes her a relationship. The series follows his attempts to protect everyone while navigating an increasingly tangled web of misunderstandings, all set against the backdrop of a Japan on the verge of ruin.