Makoto Niwa is a high school boy who, since his parents are away on business, lives with his aunt’s family. It is there he meets his mysterious cousin of the same age Erio Touwa—who happens to tie a futon around her upper body and is a self-proclaimed alien. Her staple food is pizza. Erio had been missing for half a year and was found floating in the sea. She doesn't remember anything about what happened during that period of time, but she began to think that it was the act of an alien and wanders the neighbourhood wrapped in the futon.
Spring. The "season of love" has arrived and it seems that finding himself a girlfriend was harder than Junichi believed. To break the status quo, Junichi's friends have forced him into confessing to the gal, Yame Yukana. However, things do not go quite as he expected. A series of "firsts" begins!