Twenty-three-year-old Daisuke Aoki lands his first teaching job at Futatsuhashi Elementary, where he is put in charge of the third-grade Class 3-1. One of his pupils, the precocious nine-year-old Rin Kokonoe, declares herself his girlfriend and presses provocative advances on him at every turn, with her classmates Kuro Kagami and Mimi Usa egging her on. As Aoki struggles to hold proper boundaries, Rin's behavior gradually leads him toward the troubled home life behind it.