Shirou Kirishima and Akane Hayasaka are high school classmates who share a secret. To everyone around them, they appear to be an ordinary couple, walking home together and holding hands. In private, however, their relationship is founded on a pragmatic and painful agreement. Both are in love with other people who are unavailable to them. Shirou has feelings for the student council president, Hikari Tachibana, but she already has a boyfriend. Akane harbors a crush on Yanagi, an older boy who is Shirou's senior from elementary school. Unable to be with their first choices, they have settled for being each other's second choice, promising to end their arrangement immediately if either of their true feelings is ever reciprocated.

The story is set in a contemporary Japanese high school and follows the evolution of this transactional relationship. What begins as a way to fill an emotional and physical void soon becomes complicated. As they spend more time together, the line between acting like lovers and genuinely caring for one another begins to blur. Their mutual agreement to simply use each other as a substitute becomes unstable as they find themselves growing unexpectedly closer. This creates a central conflict where they must navigate their persistent feelings for their original crushes alongside the new, authentic emotions developing between them.

Several notable narrative arcs drive this tension. The initial arc establishes the secret pact and the reasons behind their unrequited loves, including the detail that Hikari is engaged to be married after graduation. As the story progresses, a significant arc focuses on the characters breaking their own rules. Even as they get closer to the people they originally longed for, they find themselves unable to stop their deepening involvement with each other. The situation grows increasingly messy and dangerous, with the characters making choices that are described as irreparable and morbid. Later arcs, which take place as the characters move on to university in Kyoto, introduce new classmates like Akira Tono and Shiori Miyamae, further complicating the web of relationships. The series is noted for its mature depiction of romance, exploring themes of lust, jealousy, and the consequences of emotional dishonesty, moving far beyond a typical lighthearted romantic comedy.