Young con man Martin Taylor is out grifting on the streets of London when he happens across Tom Bates, a middle-class family man who, Martin quickly learns, has a disabled daughter at home who's been left brain-damaged by a hit-and-run accident. Pretending to be an old love interest of the girl, Pattie, Martin soon tricks his way into the family's countryside home, where he promptly charms mother Norma and convinces her to let him stay for a few days to be close to and care for Pattie - much to the wariness of Tom, who questions Martin's intentions. As the young man ingratiates himself more and more into their family life, it soon becomes clear that he harbours a much more sinister purpose.