Fox Mulder (David Duchovny) calls Dana Scully (Gillian Anderson) on Christmas Eve to investigate a haunted house in Maryland. He tells Scully that during Christmas of 1917, a young couple was faced with death from both war and an epidemic. The couple agreed to a lover's pact, one killing the other and the remaining one committing suicide. He explains that they could not stand the thought of being alone after the other died and during the afterlife; they wanted to experience it all together. That was 81 years ago. Now, Mulder explains, they want to make couples who venture into their mansion have the same fate.
Scully is hesitant and not interested in exploring the house because it's Christmas Eve and she has wrapping to do, but she eventually relents. Once inside the house, strange things happen: the door to the mansion slams shut, there are creaks heard in the ceiling from the upper floor, there are gusts of wind, a shadow in the form of an old woman in a nightgown, and the house has a clock which keeps perfect time starts dinging at exactly midnight. They fearfully and hesitantly decide to investigate the floor above them. Scully calls their fear an irrational fear that all humans face. Mulder does not listen to Scully's analytical view; instead, he tries to find a door that will open. One finally does and Scully marches bravely into the room to discover a massive library.
Mulder and Scully investigate the room, along with its eccentric features. The floor boards start shaking, and upon cracking open a few boards, they discover two corpses that have the same clothes and hairstyles as Mulder and Scully along with two gunshot wounds. They decide to go search other rooms, only to find that every door they walk through is the same library room they first entered. They then decide to split up, hoping to find a way out of the room.
While separated, they meet the inhabitants of the mansion, Maurice (Ed Asner) and Lyda (Lily Tomlin). The ghosts soon turn the agents against each other. Scully is told that Mulder will kill her. Scully meets back up with Mulder, only for him to pull out a gun and shoot her. Scully, completely confused, loses consciousness, and the perspective shifts. It is revealed that Lyda is actually the one carrying out these actions and, through her ghostly ability to create apparitions, causes Scully to see Mulder instead. Meanwhile, Mulder comes upon a bleeding Scully lying on the floor. When he leans over her to try and help her, she shoots Mulder in the stomach. Again, the audience sees that it is Lyda pretending to be Scully, manipulating Mulder.
Both Mulder and Scully stumble down the stairs in hope of just getting outside to die. They meet up by the door, both crawling on the floor, covered in massive amounts of blood. Scully claims he shot her, while Mulder claims she shot him. Mulder realizes that that could not have happened and stands up. The illusion is broken, and the two leave the house. Maurice and Lyda sit by the fire, holding hands, saying that they almost had the two agents. Meanwhile, at Mulder's apartment, Mulder and Scully exchange gifts, even though they told each other they would not.