An emergency medical technician, Mitchum responds to a call at a mansion high up a hill. There a wealthy woman (Barbara O'Neil) has almost asphyxiated from the gas in her unlit bedroom fireplace. Was it a suicide bid, or something more sinister? Her husband (Herbert Marshall), a burnt-out novelist she supports, can't explain it. Neither can his daughter by a previous marriage (Jean Simmons).
Mitchum finds Simmons quite the dish, but she finds in him something more than a passing fancy. She jumps into her sleek sports car, follows the ambulance back down to the hospital and waylays Mitchum in a diner. Generous with his affections, Mitchum breaks a date with his steady girlfriend (Mona Freeman) in order to spend a perfectly `innocent' evening of dining and dancing with Simmons.
But his experience with fractures and coronaries hasn't equipped him to deal with a dangerously scrambled psyche. Simmons first invites Freeman to lunch so she can humiliate her by spilling all the details, cunningly tweaked up, of her `innocent' rendezvous with Mitchum. Then she arranges for him to take on the job of family chauffeur, installing him in a garage apartment (just like Joe Gillis in Sunset Blvd.). And she hits up her stepmother to lend Mitchum the money to start up his own business as a car mechanic. Telling himself that he's just looking out for Number One, Mitchum blithely lets her erase any boundaries between them.
Klaxons start bleating, however, when she pounds on his bedroom door in the middle of the night with a cockamamie story about O'Neil hovering over her bed and playing with gas again; the earlier incident, she claims, was just a smokescreen. She tells him, too, that the stepmother reneged on his loan in order to get back at her. Mitchum's wariness enrages Simmons and redoubles her delusional obstinacy.
When her father and stepmother perish in a spectacular freak accident (their car plummeted in reverse down the steep ravine abutting the driveway), the heiress Simmons finds herself charged with murder. As does Mitchum he had the expertise to sabotage the vehicle. Wily attorney Leon Ames (in a small but succulent part) sees the defendants' marriage as the path to acquittal. Which leaves Mitchum with a Hobson's choice risking either the gas chamber or the psychotic wrath of a woman he never loved....
Though Preminger can deploy twists of plot with the best of them, he had a subtler knack of keeping his audience off-balance, never quite sure in which direction the story might develop. So for a while we share the perplexity of Mitchum, so laid back that he doesn't grasp that he's playing with a five-alarm blaze until it's too late; opportunistic but lazy, he's the perfect stooge.
Simmons may have been working within her limitations in her low-voltage, passive-aggressive performance, but she fits the character, who operates in a world inhabited only by herself. She's not a duplicitous vixen scheming to get what she wants; what she wants is the only reality she knows. Preminger recognizes this, and gives her one of the movie's quietest, most freighted scenes: During one of Mitchum's flights from her, she snoops as if sleepwalking through his rooms, finally curling up in his easy chair, his sport coat draped around her shoulders against the dawn chill. It's an eerie calm before the final storm.