The protagonist, Tonomiya Masashi, wakes up one morning and realizes that he has completely lost his memories. He doesn't even recognize his own name. As he explores an unfamiliar house, he learns that he has three younger sisters, and they all live together in the house while their parents are currently traveling overseas. The story revolves around Masashi struggling with his newfound situation of living under the same roof with three unknown high school girls, as well as dealing with the sibling relationships. Will he be able to keep his cool around them? Will his memory ever come back? Only time will tell.