When private detective Joe Kenyon loses his life in a freak automobile accident, his friend, Boston Blackie, a reformed jewel thief turned private detective, offers to help his wife Helen run her husband's detective business. As a favor to Helen, Blackie and his sidekick, The Runt, take Joe's place as a guard at a costume party given by socialite Mrs. Carter, the owner of a priceless pearl necklace. Disguised as mystics, Blackie and Runt are escorted to the party by Mrs. Carter's niece, Doris Bradley. There they meet Igor Borio, Mrs. Carter's ballet instructor, and Joan Howell, Doris' neighbor, who is enthralled by the dashing Borio. While performing her ballet debut with her instructor, Mrs. Carter pirouettes, sending her pearls flying to the floor. Borio retrieves the necklace, and later, after the dance, Mrs. Carter discovers that the pearls are imitations. Inspector Farraday and Sgt. Matthews of the police department are summoned, and when Blackie finds that someone stuffed the real pearls into his pocket, he drops the necklace into a vase. After they are recognized by Farraday, Blackie and Runt flee, and in the commotion, someone slips the pearls out of the vase. Upon discovering Blackie's link to Kenyon, Farraday questions Helen and informs her that someone had tampered with her late husband's car. To elude the police, Blackie and Runt disguise themselves as Doris' elderly parents and return to her apartment. Soon after, Joan arrives, and when Blackie finds the necklace concealed in the pocket of her fur coat, she claims that she borrowed the coat from Sandra Doray, Borio's assistant. Suspecting that Borio switched the pearls during the dance, Blackie sets a trap for him. After safely locking the pearls in Kenyon's filing cabinet, Blackie hands Doris the keys and then instructs her to notify Borio that she has found the pearls. Borrowing another disguise from Kenyon's closet, Runt poses as a messenger and delivers the fur coat to Sandra. Upon returning to Kenyon's office, Runt finds a receipt book containing the combination to the safe in his jacket pocket. When Joan tells Farraday that she gave the pearls to Doris' parents for safekeeping, Doris admits that Blackie and Runt were impersonating her mother and father. Doris then takes Farraday to Kenyon's office and discovers that the pearls have been removed from the filing cabinet. Blackie, meanwhile, visits Borio's dance studio and, masquerading as an insurance investigator, offers to sell the pearls for his unnamed client. While Blackie spars with Borio, Runt searches Sandra's apartment and finds a newspaper clipping about her involvement in a jewel robbery with a fence named Louis. Suspecting that the jewel ring may be back in business, Blackie goes to Louis' pawn shop and finds Sandra there. To flush out her partner, Blackie offers to sell the pearls and arranges to meet Sandra at a cemetery later that night. Blackie then phones Farraday and notifies him to meet him at the cemetery. At the appointed time, Mrs. Carter's husband, Mason Carter, steps from the shadows, gun in hand, and demands the pearls. When Farraday appears, Carter claims that he was only trying to recover his wife's necklace, and Blackie flees. Deciding to turn the pearls over to Farraday, Blackie and Runt return to Kenyon's office where Blackie has locked the jewels in the safe. Blackie discovers Carter and Sandra trying to break into the safe, and slugs Carter, but Carter overpowers him and then notifies Farraday that he has caught Blackie trying to crack Kenyon's safe. When Farraday arrives, Blackie produces the combination, proving Carter's accusation to be false. Exposed, Carter flees and is shot by Farraday. Sandra then confesses that Carter was penniless, and as he had been denied access to his wife's fortune, he fabricated a scheme to steal her jewels. When Kenyon began to suspect Carter's criminal activities, Carter tampered with his car, sending him to his death.