Fifteen year old Ranze lives with her parents and Rinze, her younger brother, in an old castle. Her mother is a werewolf and her father is a vampire. On her first day at a new school Ranze discovers that she can shapeshift when she gets into a fight with Yoko Kamiya, one of her classmates. They start fighting because they are both interested in Shun, a classmate of theirs. Ranze bites Yoko, knocking Yoko out and unintentionally shapeshifting so that she looks just like Yoko.
Ranze gets home still looking like Yoko. Her parents doubt that she is Ranze but her brother recognizes her. Shiira, Ranze’s mother, shows her that she can change back by sneezing. Ranze’s parents take her down into the basement and show her the door to another world, where her fiancé, whom she didn’t even know existed, lives. Ranze rejects the idea that she is already betrothed, at least in part because she has a crush on Shun.