High school boy Makoto Niwa transfers to a new city, where he is introduced to his aunt, Meme Tōwa. Dreaming of living in a student home by himself, Makoto discovers the house is also inhabited by a girl wrapped up in a futon mattress, who is introduced as Meme's daughter and Makoto's cousin, Erio. After enrolling into his new school, Makoto tries to get to know Erio, who mostly speaks in confusing scientific jargon. While taking her to a supermarket, Makoto is able to undo the mattress, revealing Erio as a beautiful blue haired girl, who claims to be an investigator descended from aliens, and claiming that the Earth is being targeted.

Makoto spends the night eating convenience store dinners with Erio. The next day, Makoto rides home from school with his strange classmate, Ryūko Mifune. The following day, he gets acquainted with another classmate, Maekawa, who gets dizzy if she holds her arms above her head for more than 10 seconds. Later that night, Makoto goes with Erio to the beach, where she randomly walks into the sea, claiming she can fly. On their way back, they run into Maekawa, who reveals that Erio was a former student in their school who disappeared for half a year. Meme later reveals that Erio has no memories of the half a year she was gone and had started to believe in aliens. She also mentions Erio broke her leg while trying to fly on her bike, which caused her to start wrapping in her futon.