Akitoshi Takami is a Japanese high school student living alone with his older sister, Hitomi, after their parents' early deaths. Their relationship is strained; Hitomi treats him with cold dismissal and abuse, often commanding him like a servant. Despite this hostility, Akitoshi harbors intense sexual desires for his sister, frequently masturbating to fantasies of her and finding her physically alluring.

His life changes when Mina, a supernatural entity identifying as a shinigami, appears in his room. She announces his lifespan has ended and intends to take his soul. After Hitomi intervenes and pleads for his life, Mina offers an alternative: Akitoshi can extend his life by ten years if he fathers a child within one week. Lacking romantic relationships or sexual experience outside his fantasies, Akitoshi immediately turns to Hitomi as the only person who can help him meet this condition.

Facing imminent death, Akitoshi initiates a series of sexual encounters with Hitomi aimed at impregnation. These begin coercively, including an instance where Akitoshi rapes Hitomi, triggering an unexpected shift in her behavior from hostility towards gradual kindness and cooperation. Their sexual relationship evolves into a consensual arrangement, with both siblings actively participating in repeated intercourse throughout the week. Mina occasionally joins their sexual activities, resulting in several threesomes.

Akitoshi's efforts focus entirely on impregnating Hitomi to avoid death, culminating in explicit, conception-focused sexual acts. The narrative concludes without definitive confirmation of whether Hitomi became pregnant or if Akitoshi successfully extended his life.