When Iori Kitahara heads off to college, he decides to live at his uncle's dive shop in Izu named Grand Blue. Iori is looking forward to experiencing college life amidst the roar of the waves, the blazing sun, and his cute cousin who lives with him! However, Iori runs into a group of tough guys playing strip rock-paper-scissors, and ends up joining Peek-a-Boo, the college's diving club!
Chisato and Mahiro must pay four years of overdue fees or lose their membership in their assassin guild. They take part-time jobs, but find themselves in the middle of an armed robbery and being hunted by other assassins.
