A struggling high school student alone in the world faces mounting pressure from rent and schoolwork. Just as she falls victim to a scam that steals her last savings, a horned stranger appears, declaring himself a demon king who has crossed dimensions to protect her. He claims Seina is the reincarnation of his lost love, and despite her skepticism, she cannot shake him off, ending up with a supernatural roommate determined to guard her from every possible threat.

The story centers on Seina Yamada, an ordinary girl balancing a difficult life while living alone, who names her unexpected housemate Damon. As a powerful and lovesick demon lord, Damon wields an array of abilities from hypnosis to teleportation, and can even summon a cuddly cluster of cats to solve Seinas everyday problems. The plot unfolds in a modern world with a supernatural twist, where an ancient romance and a cycle of reincarnation collide with the mundane troubles of high school, friendships, and paying rent.

The narrative unfolds across several arcs. The first volume establishes the premise as the demon king forces his way into Seinas life, determined to keep her safe whether she wants the protection or not. A significant love triangle emerges in the second volume when Damon grows intensely jealous of Seinas handsome longtime friend Shion. Damon cannot magically solve this emotional complication, which becomes even more tangled during an upcoming school trip that promises to complicate everything further.

The conflict reaches its peak in the third volume. What begins as a routine class trip where Seina shares several kisses with Damon, deepening her feelings for him, takes a dark turn. On a nighttime stroll, Shion reveals he is part of Seinas shared history with Damon from a past life and intends to kill her to ensure she never returns again. This revelation forces a final confrontation where the only way to break the cycle of reincarnation and death lies with Damon, who must decide if he can make the ultimate sacrifice to save the woman he loves.