They say opposites attract, and for werewolves Anna and Charles, this is certainly true. Charles, the pack enforcer, is a dominant alpha - whereas Anna has the power to calm others of her kind. Now that werewolves have dared to reveal themselves to humans, it's their job to keep the pack in line.
The pressure mounts when the FBI requires Charles' assistance. He's sent on a mission to Boston with Anna, and they quickly realise that a serial killer is targeting werewolves. And that they're next on the killer's list.