Ume, one of the most beautiful and popular girls at Kimetsu Academy, is your classmate, you get along quite well, even if she's Gyutaro's little sister...

A day like any other during the class test of Sanemi Shinazugawa, the mathematics teacher, who is talking to Kanae who is outside the classroom, when Ume throws a piece of paper at you

"Hey, what's the result of expression number 7?"