Elizabeth Foster, Dulcie Gray is a very efficient secretary who effectively runs a diamond merchant firm. Her boss, David Walsh, Michael Denison inherited the business from his uncle but knows nothing about diamonds. He buys a diamond ring from a street vendor, planning to propose to her, but she tells him the diamond is paste and far from worth the £25 he paid. His embarrassment over this causes him to forgo proposing. She lays out a bold plan to rescue the firm: buy the family jewels of a duke for £70,000, though they would only have to pay £7,000 on account, and she already has a prospective buyer who may be willing to pay £90,000. Yet David is reluctant to go along with how she intends to raise the sum—mortgaging his mother’s house among the possibilities—so she quits.
She walks out of her office and straight into the middle of a smash-and-grab. Accidentally blocking the back door of the gang’s getaway car, she is forced to get inside by Joe, Bill Owen. She helps the three crooks evade the pursuing police car after being promised her release, but Johnny, the gang’s leader, Sydney Tafler, reneges and she is taken to their hideout in rural Sussex, a mansion looked after by the uncle of Basher, another thief. There she meets Arthur, Charles Farrell, the fourth member of the gang. In a bid to secure her position, she tips off a farm worker who gives them a ride on his hay wagon to the mansion, but the message she wrote on a pound note is only noticed when he tries to spend it in the pub.
Elizabeth, a prisoner until the gang pulls off another planned smash-and-grab in a few days, sets to improve their living conditions and also proves to know far more about their jewel haul than they do themselves. All her predictions as to how their future plan will fail prove to be correct, and the group—except Johnny—begins to look up to her and treat her with unexpected respect. She persuades the gang to follow a plan devised by Arthur, and to her horror they still carry out a theft against her former employer, stealing the jewellery David had succeeded in buying from the duke. Worse still, Walsh, in his usual inept fashion, had neglected to insure it. Nevertheless, after receiving Elizabeth’s message, he rescues her and the jewellery.
Years later, Elizabeth and Walsh are happily married and prosperous, having purchased the mansion where she was imprisoned, with the gang now employed as their servants.
