While preparing for their midterm exams, the students get too reckless performing parkour outside of school grounds and injure Matsukata, the elderly principal of a preschool for children unable to attend school. As punishment, Koro-sensei bans the students from studying for the midterms and instead has them work at the preschool until Matsukata recovers in exchange for him keeping the class a secret. While the other students help fix up the building, which is in a decrepit state, Nagisa mentors a bullied girl named Sakura, teaching her to find the strength to return to school. After two weeks, Matsukata returns to find the students have completely transformed the preschool into a safe and fun environment, while still managing to be a positive influence on the children. With no time left to study, the students perform badly in the midterms with the exception of Karma, who manages to place 2nd overall due to having studied over the summer. Having learnt their lesson of being careful in using their newfound strength, the students receive new equipment for their assassination efforts.