London, present day. Norman Hackett, a poorly paid assistant in a high-class jewellery store, works for Mr Stoneway. One afternoon a small-time bookmaker, Mr Lumb, brings his secretary, Miss Daviot, to the store to buy a necklace following some good trading. He chooses a necklace that Norman had dreamed about buying for Anne, a window-dresser in a neighbouring shop and the object of his affection. Lumb asks for the pendant to be altered and later that day Norman is dispatched to deliver it.

At the bookmakers Norman chats with Miss Daviot who light-heartedly explains how he could make money by taking out an accumulator bet, placing money on a series of races with the subsequent odds increasing the value of his wager. On Lumb's advice Norman decides to place a bet on the performance of one jockey, Eddie Diamond, over six races in the following three days. That night Norman borrows one pound from his mother's holiday fund and places the bet. If the jockey wins all of his races Norman will make a fortune from the bookmaker. If he loses, the bet will be forfeit.

Buoyed by Eddie Diamond's initial wins Norman tells Anne about his bet. Mr Lumb and his partner start to worry that Norman's bet, if realised, will put them out of business as they have no funds to cover such a large payout. Norman calls in sick to visit the racecourse for Diamond's second day of competition, where the jockey wins all of his races. With one race left to decide his wager, Norman meets the jockey in the racecourse café, where they share cakes and talk about the bet. Diamond tells Norman that if he could persuade the owner to withdraw the final horse then he would win his earnings to date by default.

Norman visits the owner, Mrs Dooley, an eccentric animal lover, and offers to buy Old Casper, Diamond's final ride. Meeting Anne, Mrs Dooley is touched by Norman's love and decides to give him the horse as a present.

Norman and Anne rush to the course but fail to stop Old Casper's race. They rush to the enclosure to find that Old Casper has lost the race. However, Diamond, having put on weight unexpectedly after his feast of cakes, has been assigned another horse and won the race. Mr Lumb and his partner try to flee the country, but Miss Daviot stays behind and writes out a cheque to Norman for his winnings. She had covered the company's losses with a bet of her own on Diamond. Overcome, Lumb asks Miss Daviot to marry him; she happily agrees.

Norman introduces Anne, as his fiancée, to his mother.