Boulting Brothers' grim tale of a schizophrenic son who snaps and develops homicidal tendencies. Mrs. Enid Durnley (Phyllis Calvert) is the fairly wealthy mother of two children - one a mongoloid with very little cognizance whom she hides away in an asylum, the other is Martin (Hywel Bennett). Martin doesn't function on a 'normal' level either and the dire circumstances of life with his mother and brother aid him little. Martin has the personality of a six-year-old when he is in his 'nice' phase so when, after he attempts to steal a toy and gets caught, a kindly student Susan (Hayley Mills) talks the shop manager out of pressing charges, the two become friends. Martin's cruel stepfather Henry (Frank Finlay), enraged by the shoplifting, talks Martin's mum into chucking him out. Susan's mum (Billie Whitelaw) runs a boarding house and takes him in. Martin, however, may be secretly plotting revenge on anyone who has wronged him.