At 29 years old, Kobuki Yoko is finally experiencing her first romantic relationship. She met Fumiyuki Kamisu on a dating app, and things have been progressing smoothly. However, any sense of normalcy is shattered when Fumiyuki confesses a devastating secret: he is already married, his wife has discovered the affair, and she wants to meet her. Consumed by guilt for unknowingly becoming the other woman, Yoko steels herself for a confrontation and goes to apologize to the wife, Rei.

Instead of the anger or sadness Yoko expects, Rei makes a calm and utterly shocking proposal: she asks Yoko to continue her physical relationship with Fumiyuki, specifically every Wednesday. Unable to refuse this bizarre request, Yoko finds herself trapped in an officially sanctioned affair. She begins visiting the Kamisu home every week, living by a set of strange and unspoken rules, including the existence of a mysterious room she is forbidden from entering.

The story follows this dysfunctional and tense triangle, where Yoko’s initial self-loathing and sense of punishment are gradually replaced by a growing suspicion that Rei’s motives are not what they seem. As the arrangement continues, the psychological pressure intensifies. Yoko experiences a pregnancy scare, leading her to take a test and, in a panic, seek out Rei for support before the result is confirmed negative at a hospital. The doctor at the clinic, introduced by Rei, is revealed to be Fumiyuki’s younger brother, Shitomo, adding another layer of entanglement to the situation.

The narrative eventually delves into the past, revealing the circumstances of Fumiyuki and Rei’s marriage from sixteen years prior and the heavy family obligations that bound them together. As Yoko becomes more deeply enmeshed in the couple's dark history, the series shifts from a simple story of an illicit affair into a suspenseful exploration of manipulation, hidden trauma, and psychological terror.