'Ferndale', Laburnum Grove, a quiet residential address in one of the newer north London suburbs, is ruled over by George Radfern, a genial respectable businessman whose hobby is growing tomatoes. Aside from his wife and their daughter Elsie, just engaged to her boyfriend Harold Russ, the household also includes Mrs Radfern's sister Lucy and her loud-mouthed husband Bernard Baxley - arch spongers on an annoyingly extended visit. On Sunday evening, needing £450 to buy into a business supply agency, Baxley makes a pitch for a loan to Radfern; minutes afterwards, Elsie and Harold apply for a similar amount to get Harold launched with his own business selling second-hand cars. A visit from Radfern's fellow tomato grower Joe Fletten interrupts their talk, but Radfern says he'll conclude both discussions after supper. As Mrs Radfern is out for the evening, Mrs Baxley prepares the meal.

Around the table, the talk turns to their dull, cosy suburb and the lure of adventure. Baxley proclaims he would never touch tainted money. Radfern disagrees. To amazed looks he cheerfully reveals that he no longer works in his wholesale paper business; his secret profession is making counterfeit notes in a criminal gang hotly sought after by the police. Sensing trouble ahead, Harold abruptly leaves, and his prospective bride retires upset to bed. After Radfern tells the Baxleys they are now accessories to his crimes, a policeman's knock sounds at the door; but it turns out he is only depositing Radfern's wandering dog. Mrs Radfern returns home ignorant of all upsets.

Elsie and the Baxleys endure a sleepless night. Monday's post brings a letter from Harold, calling off the engagement. Before he leaves for a business trip to Birmingham, Radfern suggests the Baxleys take Elsie into the West End, maybe to see a gangster film, and gives them two pound notes to spend. As the party sets off down the garden path, another policeman arrives asking for Radfern; he will come back later, he says. Fear increases as Elsie and the Baxleys overhear bus passengers talking about criminals and are seemingly trailed by a man with dark glasses. At a West End restaurant, Baxley frets over settling the bill with one of Radfern's possibly counterfeit notes, but the cashier accepts it without demur. Next stop is the Stoll Picture Theatre, where they make a quick exit after finding the glasses man sitting in the row behind.

While tending the front garden, Mrs Radfern, still ignorant of the revelations, politely receives a visit from Detective Inspector Slack, who fishes for information about Radfern's activities. Introduced to the inspector as they return, the Baxleys and Elsie scurry into the house, and finally inform Mrs Radfern of her husband's life of crime. She brushes it off as Radfern's joke, inspired by a thriller they have been reading, The Great Bank Mystery. Mightily relieved, Lucy brings up the loan matter again. Furious at her hypocrisy, Mrs Radfern suggests the Baxleys pack their bags and go.

She then advises Elsie to test the sincerity of Harold's feelings by keeping quiet about Radfern's joke; Harold instead finds out from Baxley as the couple finally leave the house. Together again, Elsie and Harold soon argue, and Harold walks out for the second time. Radfern comes back from Birmingham with Fletten (another in the counterfeit gang) just before Slack pays his return visit. Slack and Radfern fence words among the tomatoes in the conservatory: Slack makes an offer to drop criminal charges if he co-operates with the police. Radfern suggests he lacks solid evidence, though once the inspector leaves he phones instructions for the scuppering of plates and printing presses, and readies Mrs Radfern and Elsie for a trip to Amsterdam and a long sea voyage away from British jurisdiction.

Dustsheets now cover the 'Ferndale' furniture. As Radfern makes his separate departure, the policeman returns to ask if he'd serve as vice-president for the annual police sports meeting. Radfern happily makes a donation and walks to his freedom down Laburnum Grove - so quiet, so respectable, the street where nothing happens.