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 goes on a day out with Ryūko, who explains her stance on mysteries. Makoto relays what Meme had told him to Erio, though she seems reluctant to respond. Later, Meme tells Makoto not to get involved with Erio. Though he still wants to help her, Makoto decides to dispel her belief in aliens as the first step. He attempts this by taking her on her old bike and riding off a hill to disprove her ability to fly. The hill turns out to be a bit too steep, and they both go flying into the sea. Erio starts to come to terms with her being a simple Earthling and manages to introduce herself properly to Makoto.