Jordan Sands is an allergy-prone 17-year-old girl forced to be the woman of the house after her mother’s death. Her embittered widower father David is struggling to make ends meet while her younger brother Hunter drives his family crazy with gory pranks. Their financial woes may be relieved when they inherit their Great Uncle Dragomir Ducovic’s castle in Wolfsberg, Romania. After arriving in Wolfsberg during Moonlight Mania which honors the famous Wolfsberg Beast, they meet the strange and steely housekeeper of the castle Madame Varcolac .
Having spent most of her life as the Wolfsberg Manor housekeeper, Varcolac discourages David from selling the property, but real estate agent Paulina Von Eckberg (Brooke D'Orsay) has other plans in mind and begins dating David to win him over. The Sands’ strange visit soon goes awry when a freak lab accident in which a vial of LB-217 was broken and Jordan accidentally steps on it when she and Hunter were frightened by bats. After Hunter removes the vial shard from Jordan's foot, Jordan starts to develop a transformation into a werewolf.
Succumbing to the changes, Jordan becomes graceful, athletic, and self-confident - while her dark side is dangerous to others and the change befuddles her new suitor, Goran , the town’s young attractive butcher. In order to salvage her crush and return back to her normal self, Jordan must rely on Hunter to save her before it is too late. Hunter's occult-expert friends say that if Jordan stays a werewolf throughout a full moon night, she will remain a werewolf forever. The only way to lift the curse is to kill Jordan with a silver bullet, which Hunter refuses to carry out.
As Hunter tries to find a way to reverse Jordan's curse, he stumbles onto Varcolac, who reveals to Hunter that she knows of Jordan's curse, and has once lived with a cursed being herself: Uncle Dragomir. She reveals to Hunter and Jordan that their Uncle Dragomir was the Wolfsberg Beast, a kind and gentle man who was a brilliant scientist and humanitarian; he researched a way to reverse the werewolf curse so that others wouldn't be forced to live the same cruel fate he had. He led an army of werewolves into a war many years ago when a clan of evil vampires attempted to rule the world; the werewolves took the victory and the Wolfsberg country became sacred land where the Wolfsberg Manor now stands. With Dragomir dead, Varcolac fears that the vampires will try to rule the world by killing Jordan. Jordan offers to have her become the new Wolfsberg Beast to protect the castle, but Varcolac says that only a blood-line werewolf can become the protector. The three of them go into town collecting supplies for the werewolf cure.
As it turns out, Paulina is an evil vampire who wants to take over Wolfsberg Manor. It is revealed that she killed Uncle Dragomir and Jordan is next. She captures Jordan, Hunter and Mr. Sands, attempting to kill Jordan with a silver bullet. Hunter, desperate to save everyone, sights the full moon and transforms into a werewolf . Hunter and Paulina battle for a while, but Jordan saves Hunter from a blow by shoving Paulina into the sunlight. Everyone returns to Wolfsberg Manor and Varcolac cures Jordan of her werewolf curse. The lawyer in charge of Dragomir's estate allows Varcolac to continue working there, and gives the Sands the money they inherited from Dragomir.
Hunter becomes the Wolfsberg Beast to take Dragomir's place, howling in werewolf form from a mountain outside the town during the Wolfsberg festival that honors him. The family returns to the U.S. where Jordan returns to school, more secure and confident, with a new fashion sense and worshipped by the girls who bullied her previously and dates Goran, who transferred to the school. David has also begun dating again having made sure that he has seen his latest date in the day at the request of Jordan and Hunter. Unbeknownst to the Sands family, Paulina was not killed, but severely injured and in a body-cast, and moves in as the Sands' neighbor in the final scene.
Before the credits roll, the characters of the film are seen doing a karaoke of the song "Baby One More Time."
