Kei Kusanagi is a schoolboy with an obscure medical condition which causes his body to go into a coma like state during episodes of stress. At one point he experiences one of these 'standstills' for three years, during which his body didn't age. Hence though he is actually 18 he still appears to be 15. This fact is known only to a few close family members, certainly not his classmates. One night he sees a spacecraft crash into the lake, and a beautiful female alien emerge. He is even more shocked when the next day she turns out to be his class's new teacher, Mizuho Kazami, an alien who has been sent to observe Earth. She recognises him, and the pair are soon thrown together and caught by the Headmaster in what appears to be a compromising position. The only solution of course is for them to... get married.