SSNI-789 is an adaptation of the doujinshi 「家族が旅行で不在の間、娘の友達とハメまくりました」 (Kazoku ga Ryokou de Fuzai no Aida, Musume no Tomodachi to Hamemakurimashita), which translates to "While My Family Was Away on a Trip, I Had Sex with My Daughter’s Friend.". It was originally written by Yamanashi Musume (山梨娘).

The story follows a man who finds himself alone at home while his family goes on a three-day trip after winning a lottery ticket. He takes advantage of this time alone to watch an AV film, but soon someone rings the doorbell. It is Miyako Saikawa (played by Hiyori Yoshioka), a friend of his daughter.

Miyako, portrayed as a sophisticated and bold young woman, seduces the man upon her arrival by pretending to be intrigued by the film. Despite his initial hesitation and awareness of the impropriety of the situation (particularly due to his status as a father and husband), the man gives in to temptation.

This relationship quickly evolves into an intense and passionate sexual affair. Over the three days of the family’s absence, and later outside as well, the man and the young woman continue their encounters.

Over the course of the three days, their relationship becomes increasingly intimate, marked by repeated sex scenes in various settings (living room, bathroom, bedroom). Miyako cooks for him, they go shopping together, and she expresses romantic feelings, wishing to extend their relationship beyond this interlude. The man, though initially reluctant, is swept away by passion and eventually acknowledges his attachment.
There's a funny scene where they fuck while Miyako is on the phone with the man's daughter.

The story resolves with a mutual declaration of love. Miyako reveals that she came to him intentionally, not by chance, and expresses her desire to stay with him. The film ends in a studio where the man has recently moved, suggesting he has left his family. He is joined by Miyako, who seems to want to live with him, hinting at a lasting relationship to come.