High school student Kio Kakazu leads an ordinary life in Okinawa. When attending his grandfather's commemoration, a chance encounter with an eccentric girl named Eris—whose body features resemble a cat—makes him pass out. After finally waking up, Kio is surprised to discover a half-naked Eris right next to him! She explains that she is from a planet called Catia, and her duty is to research Earth and expand her kind's social network.

Due to the warm welcome she received from his family, Eris decides to live in Kio's house. However, the pair's idyllic situation is threatened when people from secret organizations—including his own childhood friend, Manami Kinjou—all aim to seize Eris. If Kio wants to leave a positive impression of Earth on Eris, he has to protect her at all costs and convince everyone that she is not a threat to the planet.