Kobayashi, an art teacher in his thirties, and Rin, the most popular girl at the high school where he works, appear to have nothing in common. In reality, they share a secret history: years ago, when Rin was a latchkey child in elementary school, Kobayashi, then a college student, would play with her after school. Before her family moved away, she made a childlike promise to marry him. Now, as a high school student, Rin has transferred to his school and is determined to rekindle their bond. She constantly concocts scenarios to get closer to him, from claiming to need massage after club activities to "accidentally" ending up alone with him in his apartment. Kobayashi, acutely aware of his position as a teacher and the age gap, struggles to maintain boundaries while being increasingly tempted by Rin's earnest and forward advances.

The story follows their secret push-and-pull relationship as Rin's teasing grows bolder and Kobayashi's resolve weakens. Early arcs focus on Rin's attempts to break through his professional caution, often leading to comedic near-misses and risqué situations. As the narrative progresses, the secret of their connection risks exposure to colleagues and friends, forcing them to navigate the social minefield of a teacher-student romance. A key turning point comes when Kobayashi finally acknowledges Rin's sincere feelings, and the two begin officially dating, though they must still hide the relationship from the school. Later chapters explore the deepening of their bond, including summer festival dates, overnight stays, and the challenges posed by external characters such as curious classmates and a suspicious student council. The series concludes with their relationship firmly established, resolving the long-running tension of the "waiting game" (the titular "suezen" concept) as they commit to a future together despite the obstacles.