Lynn Otani, a gentle and reserved girl with blood type A, is terrible at exams, vaulting, and meeting new people—especially boys. On her first day at an all-girls school, she meets Yumi (type O), Taako (type B), and Akemi (type AB), along with a university student named Taichi. They become her first friends, each with unique personalities and blood types. This lighthearted, blood-type-based romantic comedy follows the lively days of these four distinct girls as they cause a bit of mischief and explore the bittersweet excitement of first love, all centered around Lynn.