Female-fearing high school student Ooishi Kyou suddenly receives confessions from five girls on his first day at Seisei High School. Four of them are not human—they are creatures called Koibami, beings that feed on human romantic feelings by kissing their prey. Once a person has their romantic feelings devoured, they can never fall in love again. The fifth girl is a genuine human who truly loves him. Kyou cannot abandon her feelings, so he resolves to identify the real one among the five. With guidance from the biology teacher Koiyaku Raiken, who studies Koibami, Kyou learns the creatures have forked tongues and feed most efficiently on first love. He devises a plan: date each girl on a different weekday—Monday through Friday—to observe their behavior and uncover their true nature.

The five girls are Tsumori Maori, a curious and athletic girl with a positive outlook; Hinogaburi, a confident, red-haired twin-tail who is quickly revealed as a Koibami after Kyou witnesses her feeding; Mitobe Mogu, a self-deprecating otaku with low self-esteem who secretly knew Kyou in childhood; Kisaragi Musha, a shy yet aggressively forward girl raised in a strict household where romance was forbidden; and Kanze Baku, a pink-haired short bob who initially viewed love only as food but gradually develops genuine feelings for Kyou. As Kyou dates each girl, he uncovers clues, faces danger from other factions who hunt Koibami, and grapples with his own growing emotions. Notable narrative arcs include the revelation of Hinogaburi's Koibami identity early on, a summer festival where childhood connections resurface, a school trip that deepens relationships and tensions, and the exposure of a student among the newcomers who targets Koibami. The story balances suspenseful revelations with romantic comedy, exploring the fine line between affection and predation.