Kei Kusanagi isn't your typical 15-year-old high school student; he's actually 18 years old. When Kei was younger, a rare disorder left him in a deep coma that stunted his growth for the next three years. Mizuho Kazami isn't your typical teacher, either. She's actually an alien sent by the Galaxy Federation to observe the Earth. As the sole human with knowledge of Ms. Kazumi's secret, Kei is put in an awkward position: He must marry her.