Karato lives a peaceful school life where his imouto-like cousin Sakura always comes to wake him up. His parents work as researchers so there is always lab material at home. One day Sakura feeds him something from a petri dish on accident and he gets a terrible stomach ache. Later that day a string like being jumps out of Karato's belly and transforms into a naked girl at his age. The girl introduces herself as the tapeworm Parasistence Sana and promises to protect her host from all dangers. This results in funny situations as Sana doesn't have much common sense as a non-human.