The film tells the story of an unknown creature named Mr. Koro, who blew up 70% of the moon and threatened to destroy the earth in a year. He is the instructor of the least promising class in the school, Class 3E. Although Mr. Koro is a creature with Mach-level speed and strange tentacles, he does his best for his students and goes through fire and water. This makes the classroom, which was originally filled with a weird atmosphere, an unforgettable memory for Class 3E. The promise that the students of Class 3E and Mr. Koro guarded with their bond and sincere interaction also accompanies them to the future 7 years later.