An unseen dangerous patient escapes Greenwood Mental Hospital, setting off a chilling chain of events that pulls two strangers into a trap of fear and suspicion. In the aftermath, a private house is burgled and a motorist calling from a telephone box is left dead after his unattended car is stolen by a shadowy thief. The unfolding incidents knit together a nightmarish puzzle where trust is fragile and danger feels near at every turn.
On the next day, a man in his thirties, Stephen Slade, [Simon Ward], spots an appealing young woman, Belle Adams, [Hayley Mills], in a pub. When she accepts a lift from a lorry driver, he follows in his own car, a color and make (an Austin Maxi) that echoes the earlier murder scene. The lorry driver tries to force himself on her, but Belle escapes and Stephen intervenes to keep her safe. Belle wishes to catch a train at a nearby station, and Stephen drives her there; however, upon arrival he falsely claims that her train isn’t running and offers to drive her farther instead, masking his deeper preoccupations.
As the journey continues, the truth about both lives begins to surface. Through a series of flashbacks, it becomes clear that Stephen is a voyeur with troubling sexual perversions, while Belle is an orphan who endured horrific abuse at the hands of her [uncle], Peter Jeffrey, who used to watch her undress through peepholes and intrude on her bathing moments. At a lonely petrol station, the attendant, Nina Francis, is alone and watchful; Stephen takes a break to make a call, then returns to Belle as the danger outside lingers, and the attendant is discovered murdered soon after.
On the road, the couple tries to dodge police checks—first because Stephen has been drinking, then because they are harassed by two young motorcyclists. Stephen knocks one of them into the roadside, leaving the other to deal with the aftermath. To avoid detection, the pair sleep in the car. When Belle disappears briefly the next morning, Stephen assumes disloyalty and hunts for her, only to discover that she has simply been shopping for breakfast. She later ends up riding with aging Malcolm Robarts, [Sterling Hayden], before Stephen and Belle are reunited—and a newspaper headline intensifies the chase, pushing Robarts to pursue them, though he cannot catch up.
The story culminates in a hotel stay, where a deeper truth emerges: Belle’s childhood trauma, including the abuse by her [uncle], Peter Jeffrey, led to a breaking point that eventually saw her murder her abuser. Belle is arrested, but not before she kills Stephen Slade, sealing the film’s stark twist. The title is finally explained: they were both deadly strangers, bound by fear, secrecy, and a shared capacity for violence.
