At the start of the film, Ellie Andrews, a wealthy heiress, has recently eloped with a man named King Westley. Her father disapproves, and so has kidnapped Ellie and is holding her on a small yacht. There, she refuses to eat, and eventually manages to escape from the boat, diving into the water and swimming ashore. Mr. Andrews sends a group of his assistants to retrieve her, but Ellie manages to get on a night bus to New York, where she will be reunited with her husband, King. Meanwhile, snarky newspaperman Peter Warne has just been fired by his editor and is boarding the same bus.
On the bus, Ellie sits next to Peter and the two begin a rather contentious companionship. Ellie doesn't want any of Peter's help, but she doesn't know how to take care of herself very well, having been pampered and served her whole life. At one of the stops that the bus makes, Peter witnesses a man stealing Ellie's bag while she absentmindedly smokes a cigarette. Peter runs after the thief, but is unable to apprehend him. At the next stop, Ellie takes a trip to a fancy hotel and expects the bus to wait for her. Of course, it doesn't, and she is left stranded in Jacksonville. Noticing that Ellie is getting left behind, Peter gets off the bus and waits for her. She still doesn't want his help, however.
Peter and Ellie both get on the next bus to New York. Ellie still doesn't want to have anything to do with Peter, but when a smarmy and obnoxious seatmate, Mr. Shapeley, won't stop making lecherous advances on Ellie, Peter steps in and pretends to be Ellie's husband, curtailing Shapeley's creepiness. Ellie is thankful for Peter's help. Unexpectedly, the bus has to stop at a camp due to weather. Because she has no money, Ellie is left with no other choice but to pretend to be Peter's wife and share a cabin with him. In the cabin, they hang up a curtain between their beds, a divider that Peter playfully calls "the walls of Jericho." While Ellie is annoyed by the lack of privacy at first, she begins to warm to Peter. He discovers that she is a runaway heiress and sends a wire to his old editor, Joe Gordon, telling him that he has access to a story about the famous Ellie Andrews.
The following morning, Ellie is in a good mood. She goes to take a shower and she and Peter enjoy a nice breakfast. They are interrupted, however, by the arrival of Mr. Andrews' detectives, who are looking for Ellie. Peter and Ellie have to think quick, and they pretend to be a combative Southern couple, making a huge scene that discourages and tricks the detectives into leaving them alone.
They get back on the bus, and the whole bus has a sing-along to "The Man on the Flying Trapeze." Even the bus driver gets pulled into the song, but it causes him to accidentally drive off the road and into a mud ditch. The bus is stranded, and there isn't a town for ten miles. Outside the bus, Shapeley tells Peter that he recognizes Ellie from the headlines that Mr. Andrews has taken out in the papers, and that he wants to strike a deal with Peter to split the reward for spilling her whereabouts to the authorities. Peter pretends that he is a gangster to Shapeley, which discourages Shapeley from telling anyone about Ellie's whereabouts.
Peter and Ellie decide to abandon the bus, for fear that Ellie's identity will be discovered. They begin to walk through the countryside together. One night they sleep in some piles of hay, and a romantic spark begins to kindle more and more between them. Along the road, Peter lectures Ellie about how to hitchhike properly. While he has a lot of strategies, he can't seem to get any cars to stop. Ellie tries her hand at hitchhiking, simply lifting her skirt a little to reveal her leg, which immediately gets a car to stop.
They hitch a ride with a man who tries to steal their belongings at a rest stop. Luckily, Peter chases him and steals the man's car. Peter and Ellie make a run for it with a stolen car. They end up at another motel camp three hours outside of New York, where the motel owner's wife is skeptical about whether or not they are married. On their final night together, Ellie professes her love to Peter, but Peter is cold to her. After Ellie cries herself to sleep, Peter slips out of the cabin unnoticed, drives to New York and writes up a story about his and Ellie's love affair, which he then sells to his old editor, Joe Gordon, for $1000. With the money, Peter plans to drive back to the cabin before Ellie wakes up, and propose. Unfortunately, however, the motel owner's wife has woken Ellie up early and kicked her out of the cabin. Believing that Peter has abandoned her, Ellie goes to the sheriff's office and calls her father, asking him and King Westley to come pick her up.
Ellie returns to her father thinking that Peter has rejected her, which Peter in turn interprets as her rejection of him. Ellie plans to have a formal wedding to King Westley, having received her father's consent. In the weeks leading up to the wedding, Mr. Andrews notices that Ellie seems peculiarly sad, and correctly guesses that she has fallen in love with someone else. Ellie tells him about Peter, but insists that Peter doesn't love her. Mr. Andrews arranges a meeting with Peter to settle a reward for returning his daughter, but Peter doesn't want the reward; he just wants $39.60 for travel expenses. During this meeting, Peter also tells Mr. Andrews that he loves Ellie. Mr. Andrews is impressed with Peter, and on the day of the wedding, tells Ellie the story of their meeting while he is walking her down the aisle. Should she change her mind about King Westley, Mr. Andrews tells his daughter, there is a car at the gate of the house that she can flee to. At the final moment of the wedding ceremony, Ellie makes a run for it, gets in the car that her father arranged, and drives to meet Peter. Ellie and King get their marriage annulled, and she and Peter honeymoon at a cabin motel, where "the walls of Jericho" finally fall.
