From Fantasyshrine:

High school student, Kaiya Mito is currently going out with Sakaki Yuma, a young enterpriser. Out of frustration, Mito daringly threatens to cheat on the seemly nonchalant Yuma! What's more shocking is that Yuma seems unaffected by the threat and encourages Mito to do it. So during their vacation in Hawaii, a furious Mito decides to carry out his threat. However, even when he's in someone else's arms, all he can think of is Yuma. But what's the reason behind Yuma's stoic attitude? An adventurous love story.