Sōsuke Yamada, a salaryman who cherishes his family, lives a seemingly idyllic life with his wife Rika and their seven-year-old daughter Shiho. That peace shatters during a lunch break when a coworker shows him a secret social media account called Momonga Couple. The account posts explicit videos of a married couple, and in one of them, Sōsuke sees a woman who looks exactly like Rika having sex with an unknown man. The location is unmistakable: Shiho’s elementary school classroom. A mole on the woman’s arm matches Rika’s exactly, turning suspicion into horrifying certainty.

Consumed by shock and rage, Sōsuke resolves to identify the man and destroy him. He joins the school’s PTA under the pretense of involvement, quickly narrowing the suspects to five fathers who have regular access to the school: PTA president Isobe Yūta, the quiet Mori Shingo, the pushy Matsuo Jin, the charming Morisato Mitsuhiko, and the elusive Kamiizaki Subaru. Sōsuke operates in a state of paranoid vigilance, observing every interaction between Rika and these men.

His investigation leads him to an unexpected ally: Megumi Akiba, Shiho’s homeroom teacher, who secretly enjoys watching the school’s after-hours activities and already knows the PTA is a hotbed of affairs. She helps Sōsuke gather evidence. At a welcome party hosted by Matsuo and his wife Sena, Sōsuke is nearly trapped in a honey-pot scheme orchestrated by the couple to extort five hundred thousand yen from him. He turns the tables by secretly recording their confession, forcing them to back down. The incident awakens something darker in Sōsuke. He stops being a victim and becomes a predator, coldly determined to hunt the truth.

As Sōsuke digs deeper, the scale of the corruption expands. The PTA is not just a nest of infidelity; it is entangled in a black-market trade of middle-school recommendation slots. The mastermind appears to be Shirotori Yūma, a popular and trusted teacher who, behind his warm facade, allegedly uses these slots to coerce mothers into sexual relationships. A recurring figure in a horse mask is linked to Shirotori, and evidence suggests he may be the man behind the Momonga Couple account itself.

The most devastating revelation comes when Sōsuke discovers Rika’s hidden bank account containing eighty million yen. She claims not to know where the money came from, leaving Sōsuke trapped in uncertainty. Is she a victim, manipulated and blackmailed into participating? Or has she been a willing partner in the scheme all along, betraying him systematically while wearing the mask of a devoted wife?

The story follows several major arcs: the initial discovery and Sōsuke’s desperate denial; his infiltration of the PTA and the narrowing of suspects; the Matsuo trap and his psychological transformation into a calculating avenger; the unmasking of the recommendation-slot scandal and Shirotori’s role; and the ongoing crisis over Rika’s true nature. Each revelation tightens the noose around a different suspect, while the central question remains unanswered: who is the man in the videos, and was Rika acting of her own free will?

The narrative builds toward a confrontation with Shirotori, the suspected kingpin, while Sōsuke must decide whether his revenge can be satisfied without destroying whatever remains of his family. The story explores themes of trust, betrayal, the double lives people live through secret accounts, and the rot that can hide within the most ordinary institutions, all set against the claustrophobic world of school committees and suburban propriety.