Sam Winchester left hunting and his family two years ago for college, but when John suddenly shows up in his home one night to tell him Dean is missing he agrees to go with his father to help him find his brother. They find out that Dean has been held prisoner by a pack of werewolves, and they manage to rescue him.
What they don't realize is the pack leader has set her eyes on Dean as a new possible mate, and Dean has been bitten by the pack leader. Dean knows he only has two choices, to die or become a monster, and begs his father and Sam to simply kill him. Threatening to do it himself if they won't.
John and Sam know they only have until the next full moon until Dean turns to try to find a cure, and to try to keep Dean from killing himself before then. As though those weren't enough problems, the pack and their leader are tracking them down trying to retrieve her new 'mate'. And Dean is beginning to have inappropiate feelings towards his brother that he's never had before. Because wolves mate for life, and a part of him has chosen Sam.