The film starts with a dangerous patient escaping Greenwood Mental Hospital causing injury or death to two staff. Immediately afterwards a car is stolen and the owner killed by the thief.
The plot follows Stephen Slade (Simon Ward) who notices a pretty girl Belle Adams (Hayley Mills) in a bar and follows her when she is given a lift by truck driver. He is driving a car the same color and make as seen in the earlier murder scene. The truck driver attempts to rape the girl but she escapes and is rescued by Slade.
Adams wishes to catch a train at a nearby station and Slade in wishing to take her there (really wishing to exploit her) lies about delays in her train as a way of keeping her with him.
With roadblocks in the area and Slade not being too forthcoming on his own background, the trip focusses on what they are both hiding. The flashbacks gradually reveal that Stephen is a voyeurist deeply into sexual perversions, and Adams is an orphan who was a victim of sexual abuse on part of her uncle. Somebody murders a girl who was an attendant at the filling station the couple has dropped by.
On waking the next morning Slade loses Adams and hurries off to find her, where she misses him and catches a lift from aging Malcolm Robarts (Sterling Hayden). However, within a short time Slade reunites with Adams, leaving Robarts alone. Having realized something, Robarts tries to chase them but doesn't succeed.
The film culminates with the couple spending night at the hotel. In the last few minutes the truth is revealed: after massive sexual abuse Adams snapped and murdered her uncle which resulted in her having been confined to Greenwood Mental Hospital. Adams is finally arrested by the police, but not before she murders Slade. The title is explained: they were both deadly strangers.
Belle Adams - Hayley Mills
Stephen Slade - Simon Ward
Malcolm Robarts - Sterling Hayden
Jim Nicholls - Ken Hutchison
Belle's Uncle - Peter Jeffrey
Café Owner - Hubert Tucker
Petrol Station Attendant - Nina Francis
1st Motorcycle Youth - George Collis
2nd Motorcycle Youth - Ralph Arliss
Stephen's Girlfriend - Juliet Aykroyd
Motorcycle Policeman - Roger Nott
Hotel Receptionist - Norman Tyrrell
Time Out noted "old-fashioned psychopathic goings-on in the West Country" and its "sole redeeming feature is Hayley Mills, who suggests an actress capable of much better things than she has been offered recently. Hayers, to his credit, does exploit her best quality - an insolent, slightly offhand sex appeal" ; while TV Guide found it an "occasionally intriguing tale," concluding that it was "well done, but it seems to bog down in its own cleverness"; whereas The Terror Trap found it "a satisfying and well made British psycho thriller."