It Happened One Night begins with a rich heiress named Ellie Andrews sequestered by her father on his yacht, disapproving of her marriage to a famous aviator named King Westley. After an argument, she escapes the yacht and swims away. She buys a bus ticket to travel back to New York to her husband, where she meets an out of work reporter named Peter Warne. Eventually he finds out her true identity from a newspaper article about her escape. He offers to help her get to her destination in exchange for exclusive rights to her story, and secures her cooperation by threatening to turn her in to her father if she does not agree.
Andrews and Warne share hotel rooms on their trip, pretending to be husband and wife to keep from arousing suspicion. Because she is a married woman, they put up a blanket as a barrier, with Warne referring to it as the walls of Jericho. One of the other bus passengers recognizes Andrews from an article offering a $10,000 reward for her return. While the bus is stranded in mud, the passenger offers to split the money with Warne if he helps him turn in Andrews. Warne scares the passenger away by implying that hes a professional criminal looking for $1,000,000 in ransom money and then threatens to shoot him when he becomes frightened. Warne and Andrews then leave the bus to hitchhike in case the other passenger decides to turn them in to the police.
After they spend the night sleeping in hay, Andrews gets a driver to stop by flashing one of her legs, who turns out to be a robber who drives off with their possessions. After chasing after him, Warne inexplicably returns with a bloody temple and the mans car. In the meantime, the father reluctantly agrees to allow the marriage to the aviator if his daughter returns. While spending another night in a blanket-partitioned hotel room, Andrews throws herself at Warne and proposes that they both be together. Warne seemingly rejects her and after she falls asleep he drives back to New York in the middle of the night. He sells his story about his love and potential marriage to Andrews to his old boss for $1000, so as not to propose to her while broke. Meanwhile, the suspicious manager of the hotel kicks out Andrews when she finds out that Warne has left.
Thinking that Warne hates her, Andrews phones her father in order to turn herself in. Her father picks her up with a police escort while Warne is returning just in time to see them drive by. Angry with her, Warne contacts the father about reward money. He agrees to go to their home while a real marriage ceremony for Andrews and the aviator is to take place. He only comes to collect $39.60 for his expenses and not the full $10,000 and admits to the father that he loves his daughter. The father starts to like Warne and during the ceremony manages to convince his daughter that Warne really loves her and that she should leave the aviator. Andrews runs away during the ceremony, her father pays off the aviator to annul the wedding, and she eventually marries the reporter. The film ends in a hotel room with the "walls of Jericho" coming down.