Four members of the Russell family are summoned to hear the reading of the will of notorious practical joker Henry Russell. All four will receive £50,000 - but subject to various conditions.

Haughty spinster Agnes must work as a domestic servant for a month, patrician writer Deniston must behave like a character in one of his (pseudonymous) blood-drenched thrillers by committing a crime and getting imprisoned for 28 days, shy young Herbert must hold up the bank where he works, and rakish cad Simon must marry the first unmarried woman to whom he speaks after the reading - and revealing any of this forfeits the money.

Simon asks his butler Benson to find a suitably rich woman. Herbert tries to overcome his fear of his tyrannical boss Mr Wagstaff. Agnes apologises to her new employer for the lack of references, but Joan Webb is more concerned about how long she'll last with her father.

Driving in the countryside, Simon encounters Lucille Grayson, who is trying to fix her car. When she implies that she's hugely wealthy, Simon asks her out.

Deniston's fiancée Elizabeth refuses to postpone the wedding unless he tells her why. Deniston implies that he's on secret service business by claiming that the government will be paying his expenses and he might spend some time behind an iron curtain.

Agnes finds Mr Webb is unreasonably demanding, and he responds by firing her, only reconsidering after she offers him a thousand pounds for keeping her on for a month. Searching her suitcase, he discovers her implausibly upmarket address. He hires detective Roger Godfrey to investigate, though the latter takes more of a shine to Joan than the job.

Deniston attempts to get caught shoplifting, but he is repeatedly thwarted, not least by a pickpocket who removes the evidence. Deniston attempts other crimes, but when he's arrested for carrying a suitcase full of "burglar tools" the sergeant recognises him, laughs and asks for his autograph.

Lucille confesses to Simon that she lied about her wealth and status - she was on her way to meet the man her uncle wanted her to marry, but fell in love with Simon instead. Enchanted, Simon proposes.

Shaken by near-simultaneous encounters with his devoted secretary Sheila and Elizabeth - the latter breaking off their engagement - Deniston is finally spurred into breaking a jeweller's window, assaulting the arresting officer for good measure.

Joan tells Roger that she can't marry him, as she's all her father has. Agnes overhears this and implores them to marry - revealing that she sacrificed her youth for someone like Mr Webb, and received only misery and loneliness in return. She offers to look after Mr Webb full time while Joan and Roger get married. Mr Webb is furious when Agnes tells him about the wedding, and fires her.

Herbert finally plucks up enough courage to carry out the hold-up - but bursts into Wagstaff's office just after it's invaded by genuine robbers. They surrender, and he becomes a hero.

Deniston is horrified to discover that the magistrate presiding over his case is Elizabeth's father Sir Charles Robson, and discomfited further when Sheila takes the witness stand, claiming that he's suffering from strain as a result of his artistic endeavours. Sir Charles imposes a fourteen-day sentence, which Deniston doubles by insulting him.

Elizabeth tells Deniston that she is disgusted both by his deceit and by the vulgar books that he writes. She suggests he marries Sheila instead - which Deniston realises is a more appealing prospect.

Simon and Lucille are married, whereupon she reveals that Benson is her uncle. Meanwhile, Agnes is visited by Mr Webb, who asks her out to dinner. He was never interested in the money - just in someone who was able to put up with him.

The four regroup to hear a message from Henry Russell. He trusts that they gained something from the experience besides the money - because there was never any money. Agnes bursts out laughing and is joined by Herbert and Deniston. Simon fails to see the funny side until he looks out of the window and sees Lucille sitting in an expensive car holding a magnum of champagne - and then laughs loudest of all.