High school student Kio Kakazu lives an ordinary life in Okinawa. While attending his grandfather's memorial service, he faints after a chance encounter with an eccentric girl named Eris, who has cat-like physical traits. Upon waking up, Kio is startled to find a half-naked Eris lying right beside him. She explains that she hails from a planet called Catia, and her mission is to study Earth and expand her species' social network. Because his family welcomed her warmly, Eris decides to stay at Kio's house. However, their peaceful arrangement is jeopardized when members of secret organizations—including his childhood friend, Manami Kinjou—attempt to capture Eris. To ensure Eris leaves with a positive impression of Earth, Kio must protect her at all costs and convince everyone that she poses no threat to the planet.