|When a young woman, Madison, sees her ex-boyfriend, Kurt, watching her in a bar while she is with friends, she panics and runs off. The next day at work, she discovers her boss dead in his office, his body torn to pieces.
Sam, posing as a detective, investigates the body, discovering that his heart was missing and he seemed to have been attacked by a wolf. Sam and Dean discuss the case and realise that the obvious answer is that a werewolf is in town - something Dean is very excited about. They talk to Madison (and meet her neighbour, Glen) to find out more about the man. They find out that Kurt has been stalking Madison and was jealous of her friendship with her boss. The boys go off to look a little deeper. While they are checking out Kurts apartment, a security guard is attacked outside - with wounds that look like a wolf again.
Worried for her safety, they go back to Madison's apartment. Sam and Dean argue over which of them stays with her, while the other one heads out to look for Kurt. They play Rock, Paper, Scissors - Sam wins and Dean leaves. Sam, however, is uncomfortable - something that Madison notices and winds him up about. They end up sitting together on the sofa watching soap operas and chatting about her relationship with Kurt. She tells Sam that she was mugged a month earlier and used that as an excuse to rebuild her life. Eventually, Madison heads off to bed, while Sam watches some more TV.
Dean has been following Kurt for a few hours when he hears breaking glass in Kurt's apartment. He goes up to investigate and finds Madison, who is a werewolf, attacking Kurt. He tells Sam, who realises that it's true and locks himself and Madison in her apartment. She tries to convince him that she's not a werewolf. Dean and Sam suspect that she just doesn't know that she is changing. Rather than kill her, Sam tries to cure her - he wants to find the werewolf that bit her. It turns out that the mugger that attacked her had also bitten her and infected her. Sam agrees to stay with Madison in case she changes while Dean goes to look for the other werewolf.
It doesn't take long before Madison changes and attacks Sam, but he manages to lock her in a closet until it is over. Dean, meanwhile, has found the original werewolf and kills it while it is attacking another young woman. He takes a look and discovers that the werewolf was Madison's neighbour, Glen. When Madison wakes up, she sees the scratches and damage she caused as a werewolf and finally believes it was true. Sam and Dean, believing that they have saved Madison, leave.
Madison finds the boys lurking outside her apartment, hoping that she doesn't turn again. She invites them back in and they all wait together.When the sun rises and she still hasn't turned, they can all relax. Dean leaves, while Madison and Sam give in to the attraction between them. Unfortunately, Sam wakes up during the night and Madison has turned again - Dean suspects that she only turns when she is asleep. He tries to convince Sam that there is no other way - they need to kill her.
When Madison calls Sam in tears, they pick her up and she wonders what she did during the night. Dean confesses to her that there is no cure for her. She accepts the fact and asks Sam to kill her. He refuses, unwilling to say goodbye to another person he has feelings for. She begs him to do it and save her. Sam still refuses and leaves the room. Dean follows and they talk about it. Dean offers to be the one to kill her, but Sam accepts that he needs to do it - she asked him.
We watch Dean's face as he listens to the gunshot coming from the other room.