Kouta has been experiencing serious sexual frustration since having a baby with his wife Chiaki. While visiting his wife's family, he becomes entranced by his curvaceous sister-in-law, Chisato. One night, Kouta overhears Chisato masturbating in her room while her husband is away, and he starts to masturbate as well. When they both reach climax, Kouta accidentally raises a voice...

[Translated from the official site]