The story begins with the protagonist, Naoki Toshiya, discovering his girlfriend, Tsukimiya Airi, cheating on him with his best friend, Nakajo Shintaro. The revelation is devastating, especially since Airi is his childhood friend and his first girlfriend, and their relationship had only lasted three weeks. Overwhelmed by despair, Toshiya retreats into his apartment, wishing to disappear.
His solitude is interrupted by persistent ringing of his doorbell. It turns out to be Rinka, Shintaro's younger sister, who is a high school freshman. Rinka, noticing Toshiya's dejected state, enters his apartment and reveals that she also witnessed the affair. She expresses her anger and proposes a plan for revenge: she will become Toshiya's "affair partner" to make Airi and Shintaro feel the pain of betrayal. Toshiya is shocked by her proposal, but Rinka confesses her long-held feelings for him, explaining that she has hidden them because she knew he liked Airi. She is determined to make them pay for hurting him.
The next day, Toshiya and Rinka begin their "mission" to gather evidence. They hatch a plan to encounter Airi and Shintaro during their commute to school, aiming to create a situation where they appear to be together. During their encounter with Airi, Rinka impulsively tries to assert dominance, much to Toshiya's chagrin. Airi, however, remains possessive of Toshiya, further fueling Rinka's anger. Toshiya, witnessing their interactions, feels a growing detachment from Airi.
Later, Rinka messages Toshiya to meet her. They discuss their strategy, which involves subtly "flirting" to provoke a reaction. Toshiya is hesitant, feeling that his feelings for Airi are fading, but Rinka insists on pursuing the revenge plan. She also reveals her own complex feelings, acknowledging their unusual situation and her desire to be more than just a tool for his revenge.
The narrative then shifts to Toshiya's interactions with Shintaro, who remains oblivious to the situation and exhibits his extreme sister complex, even suggesting counting Rinka to fall asleep. Toshiya also has a brief, seemingly normal interaction with Airi, but the lingering doubt about her betrayal gnaws at him.
Rinka continues to push for evidence, and they begin staking out Airi and Shintaro. After three days of fruitless investigation, Toshiya starts to doubt the initial sighting, believing it might have been a mistake. However, Rinka remains determined, urging him to face the reality. Toshiya, touched by her dedication, confesses that he might have been better off dating her from the start.
Their investigation leads them to a family restaurant where they overhear Airi and Shintaro discussing their relationship. It becomes clear that their affair is not a simple act of betrayal but a complex arrangement involving Shintaro's resentment towards Toshiya and Airi's desire for a slow-paced relationship with Toshiya while seeking temporary gratification elsewhere. This revelation solidifies Toshiya's emotional distance from Airi.
To distract himself from the painful conversation, Toshiya and Rinka decide to enact their "Chicken Race" plan, involving discreet displays of affection. Their attempt to go to the restroom is fraught with peril as they must pass by Airi and Shintaro, and Shintaro's uncanny ability to sense Rinka's presence creates a tense moment. They narrowly avoid detection by hiding in the restroom stall together. The chapter ends with Toshiya and Rinka continuing their plan, the true nature of Airi and Shintaro's relationship still unfolding.
