Rebecca is a six and a half year-old girl who is out of luck – her parents are on the verge of divorce and she is always getting sick due to a weak immune system. One rainy day Rebecca's luck changes when she meets a magical microbe named Ernest, who becomes her best friend! Ernest is no regular germ—he's about the same height as Rebecca and he can talk. Can Rebecca keep her parents together with the help of her new friend? And can anyone really have a friend who belongs in a Petri dish?