Saeki is an ordinary high school boy with average looks and grades, who suddenly finds himself in a relationship with Sara Saionji, the heiress of the vast Saionji Group. To everyone else, Sara appears to be a perfect young lady—beautiful, intelligent, and coolly aloof, earning her the nickname "absolute zero." But their relationship is built on a secret: it is a fake arrangement. Sara, tired of constant advances from other boys, asks Saeki to pose as her boyfriend simply as a shield. He reluctantly agrees, and the two begin acting as a couple in public.

Behind closed doors, however, Sara takes the act much further than Saeki expects. She prepares homemade lunches for him, initiates physical closeness, and makes deliberately seductive moves that blur the line between performance and genuine feeling. Saeki struggles to keep his distance, constantly reminding himself that their bond is only temporary and transactional, while Sara seems to be actively trying to break through his defenses. Their school life appears ideal to classmates, who envy Saeki for landing the school's most admired girl, but both carry private uncertainties about where the charade is leading.

The story unfolds at a prestigious private academy, where Sara's family name commands respect. The early volumes focus on establishing the fake relationship's rules and the slow, awkward process of two very different people sharing intimate moments—whether it is a festival date in yukata, an afternoon at Sara's mansion, or a scene where she wears a maid uniform for him. The narrative mixes romantic comedy with explicit moments, using the fake premise to explore growing emotional vulnerability.

A major turning point arrives with the introduction of Ayaka, Sara's aunt, who becomes the academy's new chairperson. Ayaka is sharp and observant, and her presence raises the pressure on the fake couple. She scrutinizes their relationship and tests Saeki's resolve, hinting at the complex power dynamics within the Saionji family. Around the same time, Saeki's childhood friend Kurumi appears. Unlike Sara's cool demeanor, Kurumi is openly affectionate and boldly declares that she will pursue Saeki even if he already has a girlfriend; she proposes a "public two-timing" arrangement, throwing the trio into emotional chaos. Saeki is torn between his growing, confusing feelings for Sara and the straightforward warmth Kurumi offers.

As the story progresses into the third volume, a mysterious element called the "Red Secret" surfaces, connected to Sara's past and a traumatic event from her childhood. This secret is tied to why she wears a perfect mask and why she initiated the fake relationship not merely as a convenience but as a kind of experiment in connection. The cultural festival becomes a stage for revelations and confrontations, forcing Saeki to decide whether he can accept Sara's hidden vulnerabilities and whether their relationship can evolve from a convenient lie into something real. Each volume deepens the tension between the safe fiction they maintain and the messy truth of their emotions.