Frost investigates the disappearance of 8 year-old Tricia Martin. The last person to see her alive was 19 year-old Billy Conrad who has Downs Syndrome and has the mental capacity of a child. He regularly played with children of that age and seems to be keeping a secret. Supt. Mullett is concerned about the number of car being taken by joy riders and expects Frost to do something about it. Frost has to deal with a new DCI who's idea of teamwork comes up against his own more individualistic style.