High school boy Yuuki Rito is too shy to confess to the girl of his dreams, Sairenji Haruna. One day after coming home and sulking in the bath, a mysterious nude girl appears out of nowhere. Her name is Lala and she comes from the planet Deviluke, where she is the heir to the throne. Her father wants her to return to her home planet so she can marry one of the husband candidates, but she decides that she wants to marry Rito in order to stay on Earth.
Commander Zastin has been ordered to bring Lala back and has already battled Rito. He reports to the emperor that Rito would be suited to marry Lala, after hearing Rito say that marriage is impossible unless it is with the person you love (Rito was actually defending himself, but Zastin and Lala thought he was defending Lala). With this speech, Lala truly falls in love with Rito and decides that she wants to marry Rito. Her father decides that, if Rito is able to protect Lala from her fiancés, then he can be her fiancé, but if Rito cannot do so, Lala's father will kill Rito and destroy Earth.