A young woman named Millicent Barnes is waiting in a bus depot, apparently in Ithaca, New York, for a bus to Cortland, en route to start a new job in Buffalo, New York. Upon looking at a wall clock she notices the bus is late. She walks up to the ticket counter to ask the ticket agent when the bus will arrive, and he gruffly replies that this is her third time there. Millicent denies this. While speaking with the ticket agent, she notices a bag just like hers in the luggage pile behind her. She mentions this to the ticket agent, who says it's her bag. She doesn't believe this until she notices her bag is not beside the bench anymore. Later she goes into the restroom to wash her hands and the cleaning lady there insists this is her second time there. Again, Millicent denies this. Upon leaving the restroom, she glances in the mirror and sees, in addition to her reflection, an exact copy of herself sitting on the bench outside.
A few moments later she meets a young man from Binghamton named Paul Grinstead, who is waiting for the same bus. They talk about the phenomenon. Paul, attempting to calm Millicent, says it is either a joke or a misunderstanding caused by a look-alike. Just then the bus arrives and the two prepare to board it, but just as they start to, Millicent looks in the window and sees the copy of herself, seated already upon the bus. Shocked, she runs back into the depot while her identical counterpart has a facial expression of malicious indifference and something of enjoyment.
Millicent lies unconscious on a bench inside the depot while Paul and the cleaning lady attend to her. Paul agrees to wait for the 7:00 bus. While they wait, Millicent, now coming to, insists the strange events are caused by an evil double from a parallel world - a nearby, yet distant alternate plane of existence that comes into convergence with this world by powerful forces, or unnatural, unknown events. When this happens, the malevolent impostors enter this realm. Millicent's doppelgänger, evil in nature, can survive in this world only by eliminating and replacing its good counterpart - Millicent herself.
Paul says the explanation is "a little metaphysical" for him, and believes that Millicent's sanity is beginning to unravel. Paul tells Millicent he'll call a friend in Tully who has a car and may be able to drive them to Syracuse, but, in reality, he calls the police. After Millicent is taken away, in an overly direct and presumptive style by two policemen, Paul begins to settle himself. After drinking from a water fountain, Paul notices that his valise is missing. Looking up towards the main doors, Paul notices another man running through them and away from the bus depot. Pursuing this individual down the street, Paul discovers, to his absolute horror, that he is chasing his own copy, its face a mask of excited, evil delight.
As his evil copy runs away and vanishes, Paul now begins his own descent into a Hellish combination of madness, wonder and uncertainty.