One day, Kei Kusanagi spots a strange, glowing woman by the lake. He discovers that she is actually an alien officer from the Galaxy Federation named Mizuho Kazami, and even more shockingly—she is also his teacher! Despite this setback, he and his teacher gradually get to know each other, and a romance blossoms between them. However, soon enough, the school principal finds out about their relationship. The solution? Get married! Together, the newlyweds must keep their marriage a secret, not only from Kei's high school friends but also from the Galaxy Federation. Onegai☆Teacher is the fragile but comedic love story of an alien and her student.