A young girl fell in love with a young boy and made a promise with him. But as the years passed, he soon forgot about their promise. On his graduation day, Kaoru receives a ticket to a mystery tour aboard the luxury cruise ship Trident, along with his childhood friends Chihiro and Akane. He thought it was suspicious, but he decided to go anyway. When he boards the ship, he meets his classmate Kiriko and notices that he knows most people there.

[From Hau~ Omochikaeri!]