Makoto Niwa carefully tracks the balance of positive and negative experiences in his life, almost as if he's grading his own existence. When his parents leave for a trip abroad, he relocates to a new town to live with his aunt, embracing the change and eager for a fresh start. He expects his teenage years to be uneventful and typical, but he never anticipated the existence of a mysterious, long-lost cousin.
Upon settling into his aunt's home, Makoto meets Erio Touwa, a cousin he never knew he had. Though they're the same age, Erio is anything but ordinary. She spends her days wrapped in a futon, avoiding school, and insists that she's an alien. With her peculiar speech and eccentric behavior, Makoto's hopes for a normal adolescence are quickly dashed.
As he encounters a series of other quirky girls in the town, Makoto begins to wonder if his youth is heading for disaster. Yet, he may soon discover that an unconventional adolescence has its own unexpected thrills.