The narrative begins with the protagonist, Kawai Saori, a 30-year-old financial planner, discovering her husband, Maeda Mitsuo, who has been on a year-long solo assignment, is having an affair. Her suspicions were initially raised by a friend, and a private investigator's report confirmed Mitsuo's infidelity with a 26-year-old married woman named Kawaguchi Fumika. Saori, who had a marriage of convenience through an arranged introduction, feels surprisingly little shock, attributing it to a lack of deep love for Mitsuo. She immediately decides on divorce, seeing it as an opportunity for a fresh start, and plans to handle the legal and financial aspects with her professional knowledge and connections. She also contemplates how to break the news to her parents, who had encouraged the marriage.
Meanwhile, Fumika's husband, Maeda Mitsuo, receives a call from Saori, who offers to share evidence of his wife's affair. They arrange to meet at a lawyer's office. Mitsuo is devastated by the photographic evidence of Fumika with another man, realizing he had been oblivious to her infidelity for years. Saori, however, proposes a peculiar plan: Mitsuo should send a registered letter to his wife's lover, demanding a settlement. She will provide him with all the evidence. This "test" is intended to gauge Fumika's true intentions and is part of Saori's strategy to facilitate her own divorce and potentially reconcile with her husband. Mitsuo agrees, and they begin working with a lawyer.
The perspective then shifts to Mitsuo, who is revealed to be the lover. He views his affair with Fumika as a casual fling to relieve his loneliness during his solo assignment. He met Fumika a year prior when she was looking for a property, and he used his charm and manipulation to seduce her. He believes Fumika is easily swayed and dependent on him. He maintains a facade of a happy marriage with Fumika, even as he continues his affair.
The situation escalates when Mitsuo receives a registered letter from a lawyer's office, informing him of his wife's affair and demanding a settlement of 2 million yen. He is enraged, believing Fumika is trying to extort him, and struggles to find the money. He contacts Fumika's husband, who is revealed to be the same Maeda Mitsuo from the earlier perspective.
In a tense meeting with Saori and her lawyer, Mitsuo is pressured to agree to the settlement and divorce Fumika. He attempts to negotiate a reduction, but Saori's lawyer is unyielding. Mitsuo eventually agrees to the terms, realizing he has no other option. He returns home to Fumika, feigning ignorance and claiming he needs to move out of his rented apartment, which requires a significant sum of money. Fumika, eager to maintain her relationship with Mitsuo and believing her husband is unaware, offers to provide the 2 million yen from her husband's accounts, which she manages.
Following this, Fumika is confronted by Saori at her apartment. Saori reveals she knew about the affair and the money transfer, exposing Mitsuo's deception. Saori then confronts her own husband, who admits to knowing about her affair for about a month. In a dramatic turn, Saori declares she wants a divorce, and her husband, realizing his actions have consequences, agrees. Fumika, devastated and realizing she was used and manipulated, is left with debt and a broken heart.
The final chapters detail the aftermath. Saori, having divorced her husband, is now remarried to Mitsuo, who has also divorced Fumika. They have started a new life together, with Saori having founded her own company. Fumika, after a difficult period, has moved away and is trying to rebuild her life, having learned a harsh lesson about love and manipulation. The narrative concludes with Saori reflecting on her choices and embracing her new life with Mitsuo, revealing she is pregnant.