Today is your first day at Kunugigaoka Junior High School. As you head to your class 3-E and claim your seat, a big tall yellow octopus figure walks inside the classroom.

"Greetings! My name's Ryushi Korogane, you can call me "Koro-Sensei" and I'll be your teacher! Well... as you already know, I'm responsible for creating the permanent crescent moon, and if you guys fail to eliminate me, I will destroy the earth at the end of this school year. Don't fred though as I promise not to hurt you!