Lina McLaidlaw (Joan Fontaine) is a prim, proper English lady, and Johnnie Aysgarth (Cary Grant) is a suave, charming playboy. They're as surprised as anyone when they fall madly in love with each other, but since Lina's stuffy parents would never approve of her romance with a cad like Aysgarth, she must elope. Setting up a lovely home after a whirlwind honeymoon on the Continent, Lina begins to see another side of Johnnie; his charm and bonhomie hide the fact that he's not only broke, but a very accomplished liar.
Worried about their future, Lina presses Johnnie to find employment, and when he comes home one day showering expensive gifts on her, she believes his story of working for an obscure cousin, managing his estates. Dropping in one day at the office of this cousin, Lina is shocked to find that Johnnie was discharged weeks ago, and is told that she need not worry, as his former employee will not prosecute, at least not at this time.
Johnnie always manages to allay Lina's fears with his charm, but she trusts him less and less. One of his old school chums, 'Beaky' Thwaite (Nigel Bruce) has dropped in for a long visit, and Lina begins to suspect the the two of them are cooking up some scheme to cover Johnnie's gambling debts. Beaky has money but little business sense, while Johnnie is always thinking about making money without actually working for it. They have a plan to buy some rocky coastline property and develop a seaside resort, but Johnnie changes his mind at the last minute, so he and Beaky must go to London to dissolve their business arrangement.
Lina's suspicions get the better of her as she imagines Beaky meeting an untimely death on the dangerous road along the coast. Johnnie has been reading lots of murder mysteries, intrigued with the techniques of poisoning someone without leaving a trace. When the police come calling a day later, she learns that Beaky has indeed died mysteriously, and when Johnnie returns she finds she can no longer trust him. She cannot even stay in the same room at night with him, and when he brings a simple glass of milk to her before bed, Lina is filled with dread. She fears that Johnnie now plots to poison her, and collect the money from her life insurance.
Trying to get away while she still can, she tells Johnnie she must visit her Mother. Johnnie is angry with her, but insists on driving her down the same dangerous coastal road, lined with its jagged cliffs.They speed along, Johnnie drives faster and faster, skirting the edge of the cliff, and veering at the last second down a "shortcut". Lina can only ride along in terror of what Johnnie has planned.
As he swerves around a curve, Lina's door falls open and she nearly tumbles out over the edge of the cliff. Screaming, she clings to the car, as Johnnie's hand reaches over. Siezed with terror, she tries to avoid being pushed out, but he lunges at her, and at the last instant pulls her back into the car.
Skidding to a stop, he demands to know what's gotten into her. "How much can a man stand?" he asks. Gradually, understanding comes over her, and as Johnnie explains, she realizes that he had nothing to do with Beaky's death, and that his fascination with poison is centered on himself. He meant to commit suicide rather than face his insurmountable debts, and spare his beloved Lina the shame of seeing him in prison. Clinging to him, she vows that they will face the future together, and they turn the car around for home.