Marc Stevens is a struggling low-level performer, who makes his living performing light pop ballads and easy listening tunes at retirement homes. En route to perform at a Christmas special, his van — which doubles as his home — breaks down during a storm and he is stranded deep in the woods. Lost, cold, and succumbing to the elements, Marc is rescued by a local, an emaciated young man named Boris, who takes Marc to a run-down inn.
The sole occupant of the inn is its proprietor, Mr. Bartel, an amiable old man who lives there as a hermit of sorts. Claiming to be a retired standup comedian, Bartel welcomes Marc to stay and offers to repair his van as a token of brotherhood between professional entertainers. Marc accepts the offer, but remains aloof, not speaking with Bartel about his own career or personal life.
The next morning, Mr. Bartel tows Marc's van into the front yard of the inn. Marc tells Bartel he is going for a walk, at which point Bartel suddenly becomes paranoid and aggressive, warning Marc not to go into the nearby village. Marc agrees, but during his walk he approaches a nearby farm, where he witnesses a family voyeuristically watching a teenage boy have intercourse with a calf, calling the experience "so tender". Meanwhile, rather than repairing Marc's van, Bartel snoops through Marc's living quarters and takes his mobile phone and some amateur pornographic photographs presented to Marc by a fan (Brigitte Lahaie).
That night, Bartel becomes even more aggressive, working himself into a frenzy while recalling his adulterous wife Gloria who abandoned him years before. He insists that Marc sing him a song before going to bed. The next day, Marc finds the homemade porn in the inn and realizes Bartel has been going through his things; when he attempts to call for help, he discovers that the telephone Bartel has been regularly using isn't even wired into the wall. Confronting Bartel, Marc discovers him vandalizing the van and pouring petrol over it; Bartel knocks Marc unconscious and blows the van up.
Marc wakes to find himself tied to a chair, clad only in an old dress. Mr. Bartel, now babbling, addresses Marc as if he were his wife, asking why "she" has come back after leaving him. Mr. Bartel sets about shaving one half of Marc's scalp, to "protect" him from the villagers, before forcing him into bed and cuddling next to him.
The next day, Bartel ties Marc to a tractor and takes him out into the woods to chop down a Christmas tree. Marc escapes, but ends up getting caught in a rabbit snare. He lies there prone while darkness falls, until Boris wanders by. Marc begs him for help, but Boris ignores his pleas, addressing Marc as if he were his lost dog. He sits beside Marc and pets and strokes him until the desperate Marc bites his leg, at which he goes away. The next morning, Bartel, alerted by Boris, retrieves Marc, driving him back to the inn covered by a blanket on the back of a hay truck. A pair of villagers see Boris driving the truck with something concealed under a blanket, but they take no action.
Back at the inn, Bartel chastises "Gloria" for running away, then crucifies Marc behind the inn before going into the village to have a drink at the local pub. Seemingly convinced that his wife was a "slut" who was sleeping with every man in town, Bartel warns the men drinking in the bar that now "she" has "returned", none of them can "have her". The men all appear frightened at Bartel's ramblings, but once he leaves, one of the patrons sits at the antique piano and begins to play nightmarishly discordant waltz music. Gradually, the men all get up and begin dancing with one another.
Back at the inn, Bartel brings Marc into the kitchen and they sit down for Christmas dinner. Boris arrives with a calf, convinced that it is his missing dog. Bartel gives a tearful, impassioned speech about love, togetherness, and the spirit of the holidays, before a sudden rifle shot rings out and a bullet explodes through the inn's window, killing Boris. The villagers lay siege to the inn, rushing the inn with a pig on a leash as one would employ a dog, intent on reclaiming the calf, and it quickly becomes apparent that they are also intent on raping Marc, in the shared delusion that Marc is Bartel's returned wife. The villagers mortally wound Bartel before turning their attention to Marc; one of them then briefly rapes Marc on the dining room table.
Shots are fired among the men and in the confusion, Marc manages to escape from the mob and into the forest. He spends the night running from them through the woods, coming across a cemetery with an imposing crucified Christ gravestone, aligning with the Calvaire of the title. Marc manages to elude all but one of the men, who is about to capture "Gloria" when he falls into a bog and starts being swallowed up by the mire. Crying and broken, Marc approaches the drowning man. Instead of using the man's gun against him - or making any attempt to save him - Marc watches as he sinks below the surface. Finally, just before the man's head sinks into the marsh, Marc, as Gloria, responds to his impassioned question by telling him that "she" does, in fact, love him. Within seconds, the villager is dead, and Marc is left alone in the wilderness.
