Arata Natsume is a single man in his thirties who works at a child guidance center, a profession that puts him in contact with troubled youths. One day, a boy named Takuto, the orphan of a murder victim, makes a strange request of Arata. For two years, Takuto had been secretly exchanging letters with Shinju Shinagawa, a woman known as the Shinagawa Pierrot, a suspected serial killer who was apprehended while dismembering a body. Still seeking the location of his father's severed head, which was never found, Takuto asks Arata to visit Shinju in prison in his place.

Arata agrees and goes to the detention center, but the meeting does not go as planned. Shinju, an eccentric and deeply unsettling young woman, quickly deduces that Arata is not the person who wrote the letters and moves to end the visit. Desperate to get the answers Takuto needs, Arata makes an impulsive, shocking declaration: he wants to marry her. This bizarre proposal intrigues Shinju, who accepts, instructing her lawyer, Miyamae, who believes in her innocence, to make the arrangements.

Thus begins a tense and deceptive game. Arata outwardly plays the role of a devoted fiancé, hoping to extract information about the victims remains. This strategy appears to work when Shinju begins cooperating with investigators, revealing the location of another missing body part. However, Arata remains convinced that Shinju is a cold-blooded killer and that he is her potential next target. He continues the charade under the watchful eye of Miyamae, who hopes that this engagement will help restore Shinju's memory and prove her innocent.

The story deepens as the fake relationship evolves into a psychological duel. Later narrative arcs see the tension escalate dramatically when Arata breaks Shinju out of prison, leading the pair on a desperate cross-country flight. During this road trip, away from the constraints of the visiting room, the masks begin to slip as they confront their traumatic pasts and their true feelings. Arata introduces Shinju to his estranged mother, forcing both to question the sincerity of their bond. The narrative delves into the motives behind Shinju's crimes, exploring the toxic relationships and deep psychological scars that defined her past. This journey pushes the pair toward a radical conclusion, forcing them to decide if their union will end in mutual destruction or a genuinely new beginning.