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A mild-mannered college professor investigates the life of an actor who looks exactly like him.

Adam Bell is a Toronto area History college professor. He is a rather somber man, largely because he is stuck in a routine, which includes a relationship with his live-in girlfriend, Mary. While watching a rental movie, he spots an actor in a bit part that looks like him. He becomes obsessed with finding out about this double of his. He learns that the actor's stage name is Daniel Saint Claire, whose legal name is Anthony Claire. Claire is a Toronto based actor with only a few on-screen credits, and is married to a woman named Helen who is currently several months pregnant. Adam then becomes obsessed with meeting Claire, who he learns upon first sighting that they look exactly the same, from the facial hair to a scar each has, but Claire who outwardly is more "put together" than Adam. Their lives become intertwined as Claire himself ends up becoming obsessed with Adam, but in a slightly different way.—Huggo

A man attends an erotic show at an underground club. The club is accessible by a special key and each patron has his / her own personal one. The patrons are seated around a platform in a circle and watch a woman perform a show for them. The show culminates with a naked woman on the verge of crushing a live tarantula under her platform high heel.

Adam Bell (Jake Gyllenhaal), a college history professor (specializing in dictatorships), lives a quiet, monotonous life (class, home, dinner, sex and then all over again) in Toronto. Adam teaches the dictatorships are all about control over a large population. Dictatorships meets the bare minimum requirements of the population and keep them distracted with entertainment. Some dictatorships use strategies to control knowledge and ideas. Lower, education, limiting culture, censoring information, banning all means of individual expression, are just some ways to do that. This pattern repeats itself throughout history.

Adam lives with his girlfriend Mary. Adam has an active sex life with Mary, and she is used to his style in bed. Adam is very respectful of Mary's choices in bed and if she does not want to have sex, Adam would never force himself upon her.

He rents a film, Where There's a Will There's a Way, on the recommendation of a colleague, and spots an actor who looks strikingly like himself, briefly, in the film as a bellhop. Adam notes down the names of the 3 actors who performed the role of a bellhop in the movie, taking the names from the credits.

Searching online, Adam identifies the actor as Anthony Claire, whose stage name is Daniel Saint Claire. Adam finds Anthony's professional profile on the net and the movies that he has been in. Adam rents the other two films in which Anthony has appeared and becomes obsessed with the man, who appears to be his physical twin. Immediately afterwards, Adam searches some boxes in his own house and finds a photo of someone who looks like himself, with a woman's hand over his shoulder. However, part of the photo is torn out, making the woman impossible to identify. Adam finds the name of Anthony's talent agency from his online profile. Adam goes shopping to buy clothes and goggles that look just like Anthony's.

Adam stalks Anthony, visiting his talent agency, where he is mistaken for Anthony and given a confidential envelope, which had an enveloped marked as "Confidential" inside it. This 2nd envelope had an Anthony's address on it. Discovering Anthony's apartment in Mississauga, Adam calls the home, but reaches Anthony's pregnant wife, Helen (Sarah Gadon). She also mistakes Adam's voice for Anthony's and assumes it's a joke, but Adam insists he is not Anthony. This frightens Helen, and Adam abruptly ends the call.

Adam calls Anthony's home again later and reaches Anthony, who assumes Adam is a stalker and tells him not to call again. Helen confronts Anthony about the phone call. She thinks the call is from a jealous husband of Anthony's mistress. Helen is freaked out about Adam's existence, but Anthony insists he knows nothing. Anthony walks away from Helen not wanting to get into a confrontation with her about his alleged affair. Unconvinced, Helen conducts research on Adam, discovers the college where he teaches, and finds him. Helen is visibly stunned at his exact resemblance to her husband, though Adam does not realize who Helen is. To make sure that she is not talking to Adam, Helen dials Adam's number in Anthony's presence to assure herself that Adam and Anthony are 2 separate people. Helen returns home and tells Anthony that she went to see Adam.

Anthony eventually calls Adam, and they agree to meet in a hotel room, where they discover that they are perfectly identical, even having the same scar. Anthony has a more aggressive demeanor and approach, while Adam is very measured and gentle in comparison. Adam is taken aback by Anthony's direct personality, says the meeting was a mistake, and swiftly departs. The two men begin having similar dreams, or perhaps the same dream, featuring first a naked woman with a spider's head, and later a giant skyscraper-sized spider walking among the buildings of the city where they live.

The next day, Anthony is now stalking Adam. He sees Adam's girlfriend, Mary (Melanie Laurent), whom he finds attractive. Anthony plots to accuse Adam of sleeping with his wife, and shame and manipulate Adam into letting him sleep with Mary to 'get even'. He demands Adam's clothes and car keys for a night, after which he promises to disappear forever. Adam complies. Anthony impersonates him and takes Mary to the hotel.

Meanwhile, Adam goes to Anthony's apartment in retaliation and is let inside. The building concierge desperately asks 'Anthony' to take him back to the underground sex club. Inside the apartment, Adam finds a framed photo on a shelf which looks like the one he had found earlier in his own house, but now the photo is intact, and the woman is revealed to be Helen. Adam tries to act as Anthony in front of Helen, but it appears that she recognizes his nicer demeanor. She pretends not to notice and cuddles with him. Later that night, however, Helen wakes to find Adam crying and apologizing. She tells him she wants him to stay, and they make love, which she initiates.

Back at the hotel, Mary panics during sex when she notices Anthony's wedding-ring mark and asks who he really is. Anthony claims he has always had the mark. She forces Anthony to drive her home. The two get into a fight in the car which results in a high-speed crash, presumably killing them both.

The next day, Adam dresses in Anthony's clothes and finally opens the confidential letter received earlier. He finds the key to the underground sex club, given only to select members. He resolves to go there, and tells Helen that he's going out, but she doesn't respond. As he enters the bedroom, he sees, instead of Helen, a room-sized tarantula cowering against the rear wall. Adam, with a resigned look, sighs.