Master sleuth Capt. Hugh "Bulldog" Drummond is vacationing at a cottage in the country when his peace is shattered one night by a stone smashing through his window. Soon after, two men drive up and question Drummond, who pretends to be a simple farmer. After they leave, Drummond follows them and watches as they load a man's body into their car and drive away. Once the car is out of sight, Drummond returns to his cottage and finds a claim check with the name "Victoria" printed on it and a secret code signed "A5." The next day, Drummond's intruders from the previous evening, Meredith, a respected London jeweler, and his partner Shannon, recognize Drummond from a picture in the paper and send Doris Hamilton, a trusted employee of Meredith's, to search for the cache of jewels that their victim was transporting. Upon arriving at the cottage, Doris pretends that her car has broken down on the way to her uncle's estate in Tambridge. Seeing though her ruse, Drummond offers to drive her to Tambridge and then surreptitiously slips a note to Seymour, a reporter who has come for an interview, asking him to instruct Drummond's friend Algy Longworth to meet him in Tambridge. While stopped at the Tambridge hotel, Doris confides that her brother's life is in danger and begs for Drummond's help. When Drummond recounts the events of the previous evening, Doris asks if a note was attached to the rock. Drummond answers no, but offers to wire a query about the note to his housekeeper, Mrs. Eskdale. Instead of cabling his housekeeper, however, Drummond sends a phony coded reply from Mrs. Eskdale to Hartley Court, Meredith's estate. There, Doris introduces Drummond to her "uncle" Meredith. When the cable arrives, Drummond offers to investigate and drives off, heading for Tambridge. In town, he meets Algy and Seymour and telephones Inspector McIvar at Scotland Yard. After McIvar identifies "A5" as Scotland Yard operative Richard Hamilton, Drummond and the others speed back to the cottage, where they discover that Mrs. Eskdale has been drugged and Drummond's dog shot. When Drummond finds Doris' discarded glove, he deduces that Doris must be Richard's sister and has been coerced into crime to save her brother. After sending the claim check to Scotland Yard, Drummond, Algy and Seymour drive back to Hartley Court, where Algy and Seymour stand guard while Drummond scales the estate's walls to confer with Doris. Admitting she was forced to join Shannon and Meredith's scheme because of her brother's peril, Doris explains that Meredith had been holding a fortune in jewels for a family that had betrayed Britain during the war. When Meredith decided to return the jewels, Richard, posing as a courier, intercepted them for Scotland Yard. Aware that he was being followed, Richard hid the jewels before being captured by Shannon. Hearing Algy whistle a warning signal, Drummond quickly slips out the window and disappears. The next day, Drummond visits McIvar at Scotland Yard and surreptitiously copies the code that the investigators have deciphered from the claim ticket. Later, when he examines the code with Doris and reads the word "Sambo doll," he realizes that Richard must have stashed the jewels inside a doll. Just then, Drummond notices that Shannon is shadowing them, and stuffs some fake jewels in a doll taken from the Victoria Oyster Bar. Taking the bait, Shannon kidnaps Doris and the doll, but when he discovers the jewels are phony, he accuses Meredith of switching the stones. To find out the truth, Shannon drives to the building in which Richard is being held captive, and Drummond, Algy and Seymour follow. To save Doris, Richard blurts out that the doll is being held at a check stand at the Victoria Arms hotel, after which Shannon shoots Meredith and speeds off with Drummond in pursuit, arriving just in time to slug Shannon and turn the doll over to McIvar. After McIvar arrests Shannon, Doris gratefully embraces Drummond.