Cleaver. Cleaver. Chop. Chop. First the mom and then the pop. Then we'll get the pretty girl. We'll get her right between the curl.

Martin Durnley (Hywel Bennett), who has a childish alter ego "Georgie", lives with his mother Enid (Phyllis Calvert) and a nouveau riche stepfather (Frank Finlay) he resents intensely. Enid dotes on Martin, largely because her first son Pete is a "mongol", and she is afraid that Martin may inherit a similar affliction. One day whilst shopping Martin encounters Susan Harper (Hayley Mills), an attractive student currently working as a librarian. Adopting the guise of retarded Georgie, Martin befriends Susan and an innocent friendship is initiated. However friendship soon turns to obsession and Martin embarks on a killing spree with Susan as his ultimate target.