In a disturbing movie about psychosis, Hayley Mills plays Susan Harper, a young student who tries to help a rich, emotionally ill and sinister young man, Martin Durnley (Hywel Bennett). Martin is a schizophrenic who assumes the personality of a six-year-old boy when he is in his "nice" phase. Susan talks a store manager out of pressing charges against Martin after he steals a toy duck. Martin wants to take the toy to his mongoloid brother, who is in an institution. Martin's stepfather, Henry (Frank Finlay), enraged by his shoplifting, evicts Martin despite the pleas of his mother, Enid (Phyllis Calvert). Martin, again acting like a young child, is taken in by Susan's mother, Joan Harper (Billie Whitelaw), who runs a boarding house.