Nanjou Seishiro was a new teacher in Kawakami High School that started last school year. Because he was a homeroom teacher for the class 2-4 for this year, he had to go with them in the school trip to a tropical island even though he doesn't want to. In the end he decided to enjoy it as much as he could as he quite enjoyed diving around there. But then suddenly, Kirishima Nanami, the homeroom teacher of class 2-3, approached him. Looks like she wanted something from Nanjou...