At the New York office of wealthy jeweler Bainbridge Gibbons, diamond cutter Ambrose C. Park learns that his employer has invested his entire fortune in the Blue Goddess, a gem so valuable that Lloyds of London will not insure them to cut it. Ambrose insists that Sahutsky, the senior cutter, has erred in his calculations and will damage the precious stone, and pleads for the opportunity to cut the diamond himself. Gibbons refuses, and Sahutsky, who has been sedated by a doctor in preparation for the task, begins work. However, he cannot take the pressure and, cursing his profession, walks out. Later, as Ambrose is eating his lunch in Central Park, he shows a stranger an ad he has placed in the newspaper seeking information about his parents, who abandoned him on a park bench years before. Ambrose explains that he runs the notice each year on his birthday, adding that he was given the name Ambrose Central Park at the orphanage where he grew up. That evening, while celebrating his birthday alone in a café, the tee-totaling Ambrose inadvertently gets drunk on a bottle of plum liqueur and is arrested for disorderly conduct. At the police station, Ambrose encounters slick lawyer Remlick, who assures Ambrose he can locate his family for $400. Remlick is about to disappear with the money when Ambrose remarks that he would gladly give half of his life savings to be reunited with his family. Impressed by Ambrose's modest but respectable bank account, Remlick hatches a scheme to relieve him of all his money, and enlists his shady friend Duke Fargoh to impersonate Ambrose's father. Duke persuades his former girl friend, Emily Drumman, to pose as Ambrose's mother by promising to marry her. Remlick arranges for Ambrose to meet his "family," which includes Emily's pretty daughter Maggie. To Remlick's surprise, the reunion is interrupted by the arrival of Duke's friends, mobsters Tony and Herb, who pretend to be Ambrose's uncles. When Ambrose steps out, Tony identifies himself to Remlick as notorious criminal Tony Midelli, but declines to explain his interest in the small-time scam. Over supper, Tony steers the conversation toward Gibbons and the two million dollar Blue Goddess. Ambrose then accompanies Maggie to the nightclub where she works as a dancer, and when a drunken patron gets fresh with his "sister," Ambrose fights with him and gets Maggie fired. When Ambrose returns to work, Gibbons introduces him to the famous Dutch diamond cutter Van Goosen, and Ambrose offends him by disputing his calculations on the Blue Goddess. Ambrose, who has moved in with Duke and Emily and changed his last name to Fargoh, expresses his concerns about the Blue Goddess at home. Tony urges Ambrose to bring the diamond home and cut it himself, arguing that this would be in Gibbons' best interest, and Ambrose agrees. The following evening, Emily and Maggie protest the scheme, and the thugs tie them up and hide them in the bedroom before Ambrose comes home. He arrives without the diamond, however, having decided that removing it from the office would be stealing. Tony proposes that they accompany him to the office, where Ambrose can cut the diamond before Van Goosen is scheduled to arrive. The naïve Ambrose brings his "family" to the office and, after getting them past security, prepares his equipment. Meanwhile, Emily and Maggie free themselves and attempt to call Ambrose to warn him that Tony and Herb plan to steal the diamond, but the call is intercepted by Tony. The women go to the office and burst in before Ambrose can cut the diamond, confessing their deception. In the ensuing scuffle, Ambrose sets off the alarms, then hastily cuts the diamond, separating it into two perfect halves, which he and Maggie swallow. The police arrive to arrest the criminals, and Ambrose and Maggie are taken to the hospital to have their stomachs pumped. Gibbons meets them at the hospital and praises Ambrose for his work. Ambrose is morose over the loss of his new family, but perks up once he realizes that Maggie is not really his sister. As the proud Emily looks on, Ambrose asks Maggie to marry him.