After a Fenris, or werewolf, killed their grandmother and almost killed them, sisters Scarlett and Rosie March devote themselves to hunting and killing the beasts that prey on teenaged girls, learning how to lure them with red cloaks and occasionally using the help of their old friend, Silas, the woodsman's son.

Celia, who shares mental powers with her triplet sisters, finds competition for a handsome boy with Lo, a sea monster who must persuade a mortal to love her and steal his soul to earn back her humanity.