Serafina, 12, lives secretly in the basement of the Vanderbilt mansion with her father, who repairs their machines. She has never known her mother. One night, she sees an evil man in a black cloak kidnap – and possibly kill – a little girl staying at the mansion. It turns out many children have gone missing lately. Serafina resolves to stop the man, and finds unexpected help in Braeden, the young nephew of the Vanderbilts, who befriends her despite her strange appearance and poor background.
As Serafina tries to discover the identity of the man in the black cloak, she uncovers the story of her true origins, and learns she is more unusual than she ever expected. Braeden becomes the next target and she must rescue him by defeating the adversary. In doing so, she finds out the real adversary is not the man, but the cloak itself, and it tries to seduce her to its side (a common sub-plot in GU stories). But with the help of the animals she has befriended along the way, Serafina manages to conquer the cloak and save everyone.