After curvaceous show girls Lorelei Lee and Dorothy Shaw finish their nightclub act, blonde gold digger Lorelei receives an engagement ring from her beau, wealthy Gus Esmond, Jr., much to the amusement of cynical, brunette Dorothy. Gus's father, who is opposed to the marriage, has prevented Gus from marrying Lorelei in the past, and he again intervenes. Gus had planned to marry Lorelei in Paris, and so sends her and Dorothy ahead on the ocean liner Isle de Paris , cautioning Lorelei to avoid any scandal. As the buxom beauties board the ship, the American men's Olympic team comments that neither would drown if the ship sank. Dorothy, who does not share her chum's preference for rich men, is thrilled by the handsome athletes, while Lorelei searches the passenger list for suitable men to escort Dorothy. Unknown to the women, Gus's father has hired handsome private detective Ernie Malone to spy on Lorelei. Malone develops a crush on Dorothy, and is one of several men who bribe the headwaiter for a seat at Lorelei and Dorothy's dining room table. That afternoon, after Malone engineers a meeting with Dorothy to question her about Lorelei, Lorelei is introduced to Sir Francis "Piggy" Beekman, who owns a diamond mine in South Africa. Lorelei is dazzled when Piggy's wife, Lady Beekman, shows off her tiara, for she loves to find new places to wear diamonds. That night, the companion that Lorelei chooses for Dorothy, Henry Spofford III, turns out to be a six-year-old boy. After dinner, Malone tells Lorelei that he "clips coupons," and, mistakenly believing that he is well-off, Lorelei endorses his romance with Dorothy. As the days pass, Dorothy falls for Malone, although she reprimands him for criticizing Lorelei's passion for riches. One afternoon, Dorothy sees Malone taking pictures through the porthole of her and Lorelei's cabin, and after rushing inside, discovers that Lorelei was pretending to be a goat while Piggy, pretending to be a python, was demonstrating how pythons encircle their prey. Deducing that Malone is a detective, Dorothy schemes to retrieve his film. While Dorothy occupies Malone in the bar, Lorelei searches his cabin but cannot find the film. Forced to escape through the porthole, Lorelei gets stuck, but Spofford helps her to wiggle out of her predicament. With the aid of some strong drinks and knockout drops, Dorothy and Lorelei then succeed in getting Malone's clothes and the film. After developing the pictures, Lorelei shows them to Piggy, who is so grateful for her "honesty" that she convinces him to give her Lady Beekman's tiara. After they leave the cabin, Malone is retrieving the tape recorder he had planted when Dorothy catches him. Malone assures Dorothy that his feelings for her are real, but she refuses to forgive him. Upon their arrival in Paris, Dorothy and Lorelei go on a buying spree, and when they try to check into their hotel, they discover that Gus, who has received Malone's damning report, has cancelled their reservations and letter of credit. Left on their own, the women obtain jobs at a local nightclub, and soon after, Gus visits in an attempt to reconcile with Lorelei. Although Lorelei loves Gus, she brushes him off, and outrages him with her tuneful declaration that "diamonds are a girl's best friend." After Lorelei's number, gendarmes arrive to retrieve Lady Beekman's tiara, but the jewelry has been stolen from the women's dressing room. Dorothy, wearing a blonde wig, then impersonates Lorelei in court while her friend tries to wheedle the price of a tiara out of Gus. Meanwhile, Malone, who has come to Paris to meet Esmond, Sr., deduces that Piggy has stolen the tiara and successfully retrieves it. Back at the nightclub, Lorelei convinces Esmond, Sr. that a man being rich is like a woman being pretty, and he finally consents to her marriage to Gus. Dorothy and Malone, who have also resolved their romantic difficulties, join Gus and Lorelei for a double wedding ceremony, and Dorothy advises Lorelei, "Remember, honey, on your wedding day, it's alright to say yes."