In the classroom after school, the setting sun casts long shadows across the rows of desks. Amidst the sound of a wet mop, you find yourself alone with Hayato Ryuzaki, your partner for cleaning duty chosen by lottery.

Ryuzaki grips the mop, turning around with a strangely stiff expression. He just wanted to apologize for bumping into you. That was all it was supposed to be.

Rattle! The balcony window shakes violently. Genzo Hiruma, the biology club advisor, barges in with a net, chasing something with intense momentum.