On his birthday, middle-aged Henry Perkins finds himself on the underground, eagerly returning to Fulham, looking forward to a celebratory dinner with his wife, Jean, and their friends, Vic and Betty Johnson. Everything appears to be quite ordinary until he inadvertently picks up the wrong suitcase during his journey. When he disembarks at Fulham Broadway and attempts to retrieve his gloves and scarf, he quickly realizes he's holding someone else's briefcase. To his astonishment, inside it, he discovers a staggering £735,000 in used £50 notes.
Feeling a rush of excitement, Henry heads to a nearby pub, where he secretly counts the money several times in a booth. However, unbeknownst to him, a plainclothes officer named Davenport is watching him closely, mistakenly thinking that Henry might be seeking male companionship. As Henry arrives home, Vic and Betty are just pulling up, unaware of the chaotic plans brewing in Henry’s mind — he devises a scheme to flee to Barcelona with Jean, leaving his mundane life behind for good.
As the night unfolds, Henry realizes that the owner of the briefcase will likely be tracing him by Monday morning, so he is determined to be out of sight before then. Aware that all the cash is likely tied to some questionable dealings, he feels no moral conflict about his decision. However, the situation quickly spirals out of control.
Just as Henry starts to explain the hurried situation to an unconvinced Jean, Davenport appears, wanting to have a conversation with him. The ensuing encounter leads to a series of comedic misunderstandings, involving mistaken identities that bring Vic and Betty into the chaotic mix; they are introduced to Davenport as long-lost relatives on a trip back home to Australia. Furthermore, Bill, the taxi driver Henry has called to whisk him and Jean away to the airport, complicates matters further.
Amid this confusion, the briefcase's rightful owner, dubbed "Mr. Nasty," is killed by "Mr. Big," a criminal associate, and discarded into the Thames along with the briefcase, which contains a rather mundane cheese and chutney sandwich. In the meantime, Mr. Big, who speaks only in monosyllables, starts making phone calls to the Perkins' residence, leading to incredibly tragicomic moments, especially when Bill mistakenly divulges their home address to him.
Simultaneously, a policeman named Slater arrives to inform Jean that her husband has been found dead, only to generate more confusion as Henry, who is very much alive and hiding, pretends to be his fictitious brother from Australia, Freddy. As Slater begins to tire of the chaos, Jean, who has turned from a teetotaler to inebriated, continues hitting the brandy while Davenport's true nature as a corrupt officer becomes apparent. Now posing as yet another brother, "Archie," the ongoing mix-ups continue, particularly regarding the briefcases.
As plans are being made for their supposed getaway to "Barlecona," Betty offers to accompany Henry as a travel companion, leading everyone to interpret the situation as if they are exploring new relationships. Ultimately, all four decide to go to Barcelona, with Davenport insisting on joining them as a “bodyguard” and Bill taking the role of the group’s gardener.
Things take a dramatic turn when Mr. Big arrives, armed and ready for vengeance. He threatens them with a gun, having been slightly injured in a prior car accident, leading to a fierce confrontation in the house. In the end, with the chaos reaching its peak, Henry confesses everything, including all the mistaken identities, to Slater after the Dutch crook is subdued. Once the authorities handle the situation, Henry begrudgingly returns to the ordinary routine he wanted to escape. With the chicken burned and the plans gone awry, they ultimately learn that Bill has stashed the money in one of the suitcases, allowing for a bittersweet conclusion to Henry's tangled birthday adventure.
