Itogami Island is a manmade island in the middle of the Pacific Ocean where summer never ends. A high school student at Saikai Academy, Akatsuki Kojou, is on his way home from studying with his classmates, Aiba Asagi and Yaze Motoki, when he notices a girl in the Saikai Academy middle school uniform following him.

Kojou and Asagi take Sana on board Vatler's boat, the Oceanus Grave II, to protect her from danger. But Kojou's relief is short-lived as he is soon accosted by Vatler's beautiful young maids, and his stimulation is pushed very close to the limit when he ends up sharing a bath with Asagi...