Lorelei Lee, [Marilyn Monroe], and Dorothy Shaw, [Jane Russell], are American showgirls and companions, though they could not be more different in what they value in men. Lorelei is drawn to status and the sparkle of diamonds, dreaming of a fiancé who can fund her extravagant lifestyle, especially Gus Esmond Jr. [Tommy Noonan], while Dorothy cares about looks and vigor, and places little stock in wealth.
Lorelei plans to wed Gus in France, but his stern father, Esmond Sr. [Taylor Holmes], blocks their travel and resents Lorelei without having met her. Lorelei refuses to be stopped, planning to go to France with or without him; before leaving, Gus grants her a letter of credit to cover expenses and pledges to meet her there, while Esmond Sr. hires private detective Ernie Malone [Elliott Reid] to shadow Lorelei and keep her on a short leash.
During the Atlantic crossing, Malone finds himself drawn to Dorothy, even as Lorelei becomes entwined with the male members of the U.S. Olympic team aboard the ship. Lorelei also encounters the wealthy Sir Francis Beekman (Piggy Beekman), a mine owner who fascinates her, while Lady Beekman [Norma Varden] watches with a protective, wary eye.
Malone photographs Lorelei’s moments with Beekman, but Dorothy spots him and grows suspicious. Lorelei and Dorothy hatch a scheme to narcotize Malone and search him for any incriminating film while he’s unconscious. They discover film in his pants and Lorelei makes prints, while Beekman is enticed by the idea of obtaining Lady Beekman’s tiara. Yet Malone reveals that he had planted a recording device in Lorelei’s cabin and has heard their plan. He implies that Lorelei is a gold digger, and when Dorothy rebukes him for his deceit, he admits to lying. However, Dorothy confesses that she is falling for Malone, and Lorelei scolds her for choosing a poorer man when wealth could be theirs.
The ship arrives in France, and Lorelei and Dorothy spend time shopping, only to discover that Lorelei’s letter of credit has been canceled and they are ousted from their hotel after Malone’s disclosures reach Esmond Sr. They are forced to fend for themselves by working as showgirls in a Paris revue, headlining a lavish production where Lorelei performs Diamonds Are a Girl's Best Friend. When Gus appears at their show, Lorelei rebuffs him, and Dorothy begins to consider what lies ahead. Meanwhile, Lady Beekman files charges over the missing tiara, and Lorelei is arraigned for theft.
Dorothy persuades Lorelei to return the tiara, but the jewelry remains missing from Lorelei’s box. Beekman tries to dodge responsibility when Malone catches him at the airport. In court, Dorothy disguises herself as Lorelei, sporting a blonde wig and mimicking her breathy voice to stall the proceedings. As Malone is about to unmask the impersonator, Dorothy signals in covert language that she loves him but would never forgive him if he harms Lorelei. Malone wavers, then reveals that Piggy is the one who possesses the tiara, exonerating Lorelei.
Back at the nightclub, Lorelei wins over Esmond Sr. with a sharp speech about paternal money and what a daughter deserves, and he agrees to the marriage. The story closes with a double wedding for Lorelei and Dorothy and their respective grooms, sealing their futures in style.
