In 1891, Lawrence Talbot (BENICIO DEL TORO) is a famous stage actor, based in New York City and internationally acclaimed for his turns as Hamlet and Richard II. Summoned back home to England when his brother disappears, he arrives at the family estate only after his brother's corpse has been discovered -- mangled almost beyond recognition.
It's plain right away that Lawrence and his father, Sir John (ANTHONY HOPKINS), do not get along, and flashbacks soon reveal that as a child, Lawrence came upon his father holding his mother's body, her throat slashed. Still, Lawrence is drawn to his brother Ben's fiancée, Gwen (EMILY BLUNT), who asks him to stay on and discover how Ben died, as the local police believe the killer was either a terrible beast or a very powerful, insane man.
His investigation leads Lawrence to a gypsy camp, where the mysterious Maleva (GERALDINE CHAPLIN) warns him about the legend of the beast. When he tries to shoot the werewolf that attacks the gypsy camp, Lawrence himself is bitten. The hallucinations that follow combine his childhood memories and nightmares induced by his new monstrosity. Though he's increasingly infatuated with Gwen, he decides he is a threat to her and begs her to leave.
After his first night marauding as a werewolf, Lawrence wakes feeling very powerful and very guilty. He's surprised when Sir John confesses that he is also a werewolf (though you will not be surprised). Now Lawrence has to decide how to deal with his father (a serial murderer) and his own condition. Sir John advises letting the "beast go free." But Lawrence has a strong moral sense and now that he knows his mother didn't kill herself and he is not crazy (as his father led him to believe), he can also seek revenge.