Soon after the Second World War, in the West Riding of Yorkshire, Joe Lampton, [Laurence Harvey], a driven and ambitious young man, leaves his dreary hometown Dufton to take a secure yet poorly paid post in the Borough Treasurer's Department in Warnley. Eager to climb the social ladder, he pursues Susan Brown, the daughter of a local industrial magnate, even though she has been seeing the wealthy Jack Wales. Joe manages to charm Susan, catching the eye of her father and mother, who try to derail his plans by nudging his boss to steer him toward a partner from a more suitable class, offering him a lucrative job back in Dufton as a temptation he refuses once he realizes the manipulation, and sending Susan abroad on a trip—yet Susan remains deeply smitten.
While chasing Susan, Joe also keeps seeing Alice Aisgill, an unhappy, married Frenchwoman ten years his senior who came to England as a teacher and married George Aisgill, a haughty and abusive upper-middle-class Englishman who now conducts numerous affairs. Joe believes he is merely passing the time with Alice, and Alice suggests they can be "loving friends." Their feelings for each other soon deepen, and Joe begins to lose interest in his pursuit of Susan. The relationship becomes passionate and tempestuous, and after a particularly heated argument, Joe redirects his attention back to Susan. He even manages to take her virginity, yet finds himself unsatisfied and drawn back toward Alice.
Together, Joe and Alice go away on a short holiday, and Alice is buoyed by the sense that Joe may have chosen happiness over wealth and status with her. They plan that she will ask for a divorce when they return, but when she tries, George refuses and vows to ruin both Joe and Alice socially and financially if their relationship continues. While Joe broods over this, Mr. Brown delivers the bombshell that Susan is pregnant and that he expects Joe to stop seeing Alice and marry Susan, thereby allowing Joe to come into a generous salary at his business.
Facing insurmountable obstacles to a life with Alice, Joe tells her that he will marry Susan. Heartbroken, Alice sinks into drink, and the next morning, as his co-workers celebrate the engagement, Joe learns that she drove her car off a cliff where they once went together, dying slowly over several hours. Devastated, Joe wanders away from the office to the flat where he and Alice had their trysts, only to be driven away by Alice's friend Elspeth, who shouts at him and blames him for Alice's death.
Soon after, Joe and Susan marry, and with a rich wife and a high-paying job, he appears to have attained everything he thought he wanted. As they are driven away from the ceremony, Susan’s praise of the union is cut short when she notices tears in Joe's eyes, which she misreads as sentiment and declares, “I believe you really are sentimental after all.”
