Kei is an 18 year old who retains a 15 year old body after a three year long "standstill", which is a coma-like state that occurs during a time of extreme emotional distress. Standstills are derived from an unknown - possibly psychological - disease that Kei and his friend Ichigo Morino have. He tends to be a very frail, shy boy, not often wanting to assert himself when need be.

After observing the landing of a UFO while on the bank of the nearby lake and subsequently observing the alien on the lake shore, odd things begin happening to Kei in connection with this alien. The next day, Kei is surprised to find, like everyone else, that one of their teachers quit and a quick replacement was found which just happened to turn out to be the alien - Mizuho Kazami. After a few more encounters with each other, Kei is taken to her spaceship where the whole situation goes from bad to worse. Upon striking Marie, a tiny computer helper of Mizuho's, it effectively is unable to perform as well as it seemed to work, which often puts the characters in strange situations if they can't rely on the help of Marie.

Eventually, after Kei and Mizuho are found in a locked storage room by Kei's uncle and the principal of the school, his uncle impulsively tells the principal that Kei and Mizuho are in fact married. Later, Kei and Mizuho do actually get married, though only the paperwork is done while there was no official ceremony. This whole situation is, of course, kept secret from Kei's friends as Ichigo Morino constantly tries to pair Kei up with a friend of hers, Koishi, and all the while Kei and Mizuho's relationship and attraction for each other only grows.