In a securities firm, Diamond Securities, Naoya Aizawa, a 39-year-old section chief, is known throughout the company as a devoted husband who would never cheat. He and his wife Natsuna, called Kana, a 35-year-old section chief at a different branch, married after two years of dating and have been together for a decade. Their marriage is publicly admired, but in private it has grown cold. They have been in a sexless relationship for two years, and Kana’s relentless work stress causes frequent arguments and emotional distance. Into this fragile situation steps 22-year-old new employee Noa Tachibana, a beautiful and manipulative woman who also works as a social media influencer. Noa takes an interest in the famously faithful Naoya and begins a calculated campaign to seduce him, employing a variety of tricks and traps.

Naoya initially resists Noa’s advances, but the strain of his marriage weakens his resolve. Noa’s tactics become bolder, including secret meetings and planted evidence such as a lace handkerchief left in Naoya’s car, which Kana discovers. Meanwhile, Kana finds herself drawn to her younger subordinate, Minaki, who offers her support and affection. As Naoya and Kana each drift toward emotional and physical infidelity, Noa’s game escalates, exploiting both partners’ vulnerabilities. The story unfolds as a dual infidelity suspense, with the husband and wife separately crossing lines while Noa coldly observes the destruction she causes.

The narrative arcs through increasing tension: Naoya eventually succumbs to Noa’s seduction, Kana begins an affair with Minaki, and the couple’s hidden betrayals become intertwined. Noa’s motives remain ambiguous—she seems driven by a desire to test human limits or by a personal trauma. The climax sees Minaki, obsessed with Kana, stabbing Naoya in a fit of jealous rage, leaving him in a coma. Kana, pregnant from her affair with Minaki, gives birth to a daughter, but the father’s identity remains uncertain. The final volume shows the aftermath: Naoya survives but struggles to accept the child, and the couple’s relationship is permanently scarred. The story closes with a glimpse of their life years later, highlighting the lasting consequences of every choice and the cycle of cause and effect.