Yuta, a young master at the Tsukiji Fish Market, accidentally drops his meal of mixed seafood into the Sumida River. Some time afterwards a gigantic mutated squid monster arises from the depths and begins to wreak havoc upon an awe-stricken Tokyo. Attempts by the Japan Self-Defense Forces to stop the creature prove futile. As it seems things couldn't get any worse an enormous mutant octopus monster emerges from the deep and heads into a clash of the titans with the gargantuan squid. As a last ditch effort, the government forms the "Seafood Monster Attack Team (SMAT)" and an all-new plan of attack is immediately put into action. But just as the tide appears to be turning in humanity's favor, a colossal crab monster appears, joining in the Monster Seafood Wars and plunging the world into culinary chaos.

Fuutarou Uesugi is a poor, antisocial ace student who one day meets the rich transfer student Itsuki Nakano. They argue but when Uesugi realizes he is to be her tutor, he tries to get on better terms. While trying to do so he meets four other girls.