On a rainy day after school, Kazuto Izuka sees a stray dog in a box on his way to school. Afterwards, when he tries to pick it up, the dog turns into a monster. He is saved by a girl who hits the creature with a baseball bat, but she runs off and leaves her baseball bat behind. Kazuto and his friend Masaki Maruo learn the girl is Narue Nanase, a schoolmate in a neighboring class. Narue is rumored to be an alien, but schoolmate Hajime Yagi disbelieves such a thing. With Maruo's help, Kazuto follows Narue to return the baseball bat to her and asks her out for tea while thanking her for saving him the day before. When she takes him to her place, they discover that her father Tadashi Nanase is held captive by a space ninja, who was disguised as the dog monster from before. After Narue defeats the ninja with her baseball bat, she asks if Kazuto is okay with her being an alien, and then shows him her people's starship fleet.