A week after her wealthy parents die in a plane crash, fifteen-year-old Towa Tokiwa fears for her own life. Following a secret instruction her father left behind, she moves into Residence Aeternitas, a luxury apartment building on the outskirts of Tokyo that functions as a safe haven for criminals. Inside the grounds, all criminal activity is strictly forbidden and enforced by a mysterious management, making it one of the few places where a person can be safe from assassins and bounty hunters. To blend in and gain entry, Towa pretends to be the legendary, faceless assassin known only as Skull, a figure so feared that even other criminals avoid crossing him.

Almost immediately after moving in, Towa encounters the real Skull, a man who calls himself Hajime Koga. He reveals that Towa's aunt has hired him to kill the girl in order to claim the family fortune. Because of the building's no-crime rule, Skull cannot touch Towa inside the complex, but she is now trapped in a deadly game of cat and mouse: the moment she steps off the property, he is free to carry out the hit. Towa is cornered, terrified, and alone.

Her situation worsens when the only relative she still trusts, her stepbrother, turns out to be in league with her aunt. He lures her outside with a promise of help and tries to murder her himself. Skull, who has been following Towa out of curiosity and amusement, intervenes and kills the stepbrother instead. In that moment, Towa realizes she can no longer hide or play it safe. She strikes a devil's bargain with the assassin: she will give Skull her entire inheritance in exchange for his protection until her eighteenth birthday. He accepts, finding her tenacity entertaining, and what begins as a cold-blooded contract slowly shifts into an unpredictable, high-stakes alliance.

Over the course of the story, Towa and Skull navigate a world of hitmen, corrupt relatives, bounty hunters, and the constant threat of violence. Towa, a powerless but fiercely determined young woman, must learn to survive in an environment where everyone around her is a killer. Skull, meanwhile, is an eccentric, red-haired force of nature whose motives swing between protecting Towa and threatening to finish the job. Despite his terrifying reputation and unpredictable moods, he never physically harms her, and a strange, reluctant affection begins to grow between the captive heiress and the killer who holds her life in his hands.

The narrative unfolds across two volumes, following Towa's quest for survival and revenge against her aunt, the growing trust and tension between her and Skull, and the lingering suspicion that Skull may have been responsible for her parents' deaths. The story builds toward a final confrontation where Towa must decide whether she can truly trust the man who has both protected her and haunted her, and whether she can reclaim her life and her fortune from the family that tried to destroy her.