Martin and Janet and their son David are driving to their holiday destination. The weather deteriorates, giving rise to a torrential downpour. Suddenly, a man in a yellow raincoat appears in the road and is lucky not to get run over.
Martin decides to give the man a lift. After driving for a short time, the hitch-hiker attacks Martin, plunging a long dirty fingernail into his neck. The car crashes and overturns.
Janet wakes up in hospital and finds her son, David, totally untouched by the accident. Janet is told that her husband, on the other hand, has received some bad injuries. There was an emergency operation on his throat, and he will remain mute for a number of days.
The police arrive at the hospital to interview Janet. Apparently there was a tremendous struggle between Martin and the hitch-hiker. A body was found some distance from the car and taken to the hospital's mortuary. As a formality she is asked to identify the dead body.
She looks at the body. Its face is identical to that of her husband. She tells the police that the right hand of the hitch-hiker possessed a long fingernail. They explain that the right hand was severed in the crash and probably taken away by a wild animal.
Janet packs all the remnants of the struggle into plastic bags and leaves the hospital with her son. She drives to the holiday cottage, piles the blooded remains of their luggage into a mound and burns them.
When Martin is well enough to leave the hospital, he is taken to the cottage in an ambulance. Still unable to talk, Janet gives him a small bell to use if he should need anything.
That night, exhausted and weak, Martin retires to bed. Janet soon follows him, and when cuddling up to him notices that on his right hand is a long dirty fingernail. Who is the person lying next to her, Martin or a doppelganger?...
In a frenzied state Janet decides that she must take another look at the body in the hospital's mortuary. Her fears are put to rest when she sees the atrocious state of the corpse's teeth. Martin, by contrast, had perfect white teeth.
A relieved Janet arrives back at the cottage. Martin is there - and then he smiles; his teeth are rotten. Martin then goes for Janet. Janet and her son, David, frantically try to escape in the car, but Janet has left the car keys in the cottage. While Janet attempts to fend off the marauding Martin, David makes his way to the farm.
After fleeing the clutches of a demented Martin, Janet reaches the farm and hides away in the barn. In there she sees her son - he is dead! A copy of her son is also in the barn, and he is acting in the same way as Martin. Janet escapes the barn and falls into the arms of Mr Roberts, the farm owner. Mr Roberts calls for an ambulance. Janet is taken away in the ambulance and there driving is a copy of Janet!