Author and scholar Ashley Stedman is murdered in his London mansion while studying a palimpsest, or parchment, and two miniature lead soldiers, which the killer then steals. After Stedman's daughter Cynthia discovers her father's body, her boyfriend, Philip Coleman, visits private detective Hugh "Bulldog" Drummond at his apartment. Coleman asks Drummond and his assistants, Algy Longworth and Seymour, to look at two other soldiers from the same set as the stolen ones and tells them that he has received death threats from an anonymous phone caller, who is demanding the soldiers. Drummond offers to keep the soldiers for Coleman and has it announced in the press that he has acquired antique lead soldiers. After Drummond conceals the soldiers among others he buys from a toy shop, journalist Estelle Gorday comes to interview him about the soldiers, but mistakes him for Algy and Algy for him. Drummond goes along with the charade and Estelle leaves, assuming she has interviewed Drummond, who decides to let Algy continue to impersonate him. Cynthia then tells Drummond and Algy that her father acquired the soldiers and the Anglo-Saxon manuscript at an auction. The original writing on the parchment, she explains, had been covered over, but Cynthia's father was able to restore the writing and was excited by what he had discovered. Cynthia noticed a drawing of thirteen soldiers on the parchment, and soon after, a man named Edward Vane offered her father a lot of money for the soldiers and the parchment. Estelle, who is in league with Vane, invites Algy and Drummond to her apartment to enable Vane to break into Drummond's apartment to look for the soldiers. Seymour and Coleman see Vane, however, and follow him. When they return home, Seymour informs Algy and Drummond that Coleman is watching Vane, but when they all go to Soho, they find Vane murdered and see someone leaving by a fire escape. After Cynthia identifies Vane as the man who visited her father, police inspector McIver summons all the principals in the case to his office and tells them that all the police know about Vane is that he had recently arrived from Canada. Later, Drummond, as Algy, goes to see Estelle and presents her with a bottle of of her favorite perfume, mentioning that he had smelt it the night before in a certain flat in Soho. Drummond accuses her of killing Vane but she denies it. When Drummond reveals that he knows that she is not a reporter, Estelle admits the deceit, but adds that she has no interest in the soldiers and only wanted to meet Drummond. They kiss, then Drummond locks her in her bedroom while he searches through drawers until he finds the palimpsest. Estelle finally confesses to working with Vane, but not to killing him. Later, Drummond tells Cynthia that he has researched the soldiers and has discovered that they are representations of the last Anglo-Saxon leaders at the time William the Conqueror invaded England. Harold, the last Anglo-Saxon king, gathered all the royal treasures and hid them in one of his manor houses, then documented the hiding place on parchment, and made the thirteen lead soldiers together the key to deciphering the location. Drummond has figured out in which house the treasure might have been hidden and they all go to the location, where they find a recently constructed antique shop. They pose as customers and are observed by Estelle from behind some curtains. Before they leave, Coleman notices an old fireplace among the shop's fittings, and Drummond later predicts that whoever has all thirteen soldiers will have to come to that shop. Later, Estelle, who now knows who the real Drummond is, goes to his apartment and, at gunpoint, demands the parchment and tells him that she saw him at the antique shop, which her father owns. Her father, who owns nine of the soldiers, knew about the treasure and to be able to claim it, purchased the shop. She also admits to having been present when Drummond found Vane's body but again denies killing him. Drummond then suggests that they go to talk with her father. Meanwhile, Algy and Seymour are following Cynthia, who returns, with the parchment, to the antique shop. Once inside, however, someone knocks her out. Drummond and Estelle then arrive at the shop to find her father dead on the floor. When Drummond sees Coleman arranging the soldiers on the parchment in front of the fireplace, he jumps on him, telling Algy to phone McIver as they have caught the murderer. Estelle tries to shoot Coleman out of revenge, but Drummond stops her. Drummond then finds Cynthia recovering from the blow, and she tells him that Coleman had phoned and asked her to meet him at the shop. Following the manuscript drawing, Drummond arranges the soldiers on a pedestal in front of the fireplace and triggers a door in the back of the fireplace to open up. With Algy holding a gun on Coleman, they enter a subterranean vault and find a treasure chest. McIver arrives, picks up one of the soldiers to examine it and reactivates the door just as Coleman tries to escape. The door closes on him, crushing him to death. When McIver replaces the soldier, the door reopens, allowing the others to leave the vault. Cynthia embraces Drummond, who finally admits to her that he is not Algy.