Jeff Sutton, Richard Wyler, arrives in Rio carrying what he claims is $10 million in stolen money. Checking into a hotel, he connects with Lesley, Maria Rohm, a manicurist who seems oblivious to his alleged crimes. He is pursued by minions of Sir Masius George Sanders, a crime lord who also wants Sutton's stolen millions. After spending a night with Sutton, Lesley finds a newspaper that reports Sutton's crimes. She warns Sutton that being extradited is the least of his problems, and suggests the two escape Rio. Sutton agrees and the two head for the airport. Pursued by Sir Masius's men, Sutton and Lesley are split up. Lesley is captured by Sir Masius's men, while Sutton escapes by boarding a nearby plane. Thinking he's safe, Sutton realizes that the plane's only other passengers are heavily armed women in distinctive uniforms.
Brought to Sumuru, Lesley is identified as an operative of Sumuru, who has plans of world conquest. Knowing that Sutton has been captured by Sumuru, Sir Masius has Lesley tortured to reveal Sutton's whereabouts. Knocked out by gas, Sutton awakes to find himself in the secret city of Femina, where Sumuru is assembling an army of women. She personally gives Sutton a tour of the city, including a display of her wealth with which she will finance her global conquest, money she has stolen from men who themselves stole or obtained money through corrupt means. Now she has her sights set on Sutton's stolen $10 million. Among Sumuru's prisoners, Sutton recognizes Ulla Rossini, a beautiful and rich woman who refused to join Sumuru's army, Marta Reves. Jeff warns Ulla to be ready, that he was sent to Rio to get her out of Femina. Sumuru tortures Jeff to reveal the location of the hidden millions, using a radiation machine that causes a slow and painful death. When Sumuru begins torturing Ulla, Sutton breaks, revealing that there is no money, it was just a ploy to grab her attention. Enraged, Sumuru leaves Jeff and Ulla alone. Tricking the guards, Jeff and Ulla escape. Overpowering Sumuru's sentries, the two steal one of Sumuru's planes, and fly to Rio.
Reaching Rio, Sutton and Ulla are soon captured by Carl, Macius's chief henchman, and brought to Sir Masius himself. Like Sumuru, Macius is upset to learn that Sutton's money is nonexistent, then realizes that Sutton can still prove useful. As Rio readies itself for Carnival, Sir Masius has Lesley carry a message to Sumuru, offering Sutton in return for half of her fortune. Entering Rio in costume, Sumuru and her soldiers capture Ulla and Irene, Sir Masius's accountant, and order Jeff to surrender himself to save their lives.
Turning himself over to Sumuru, Sutton is again subjected to Sumuru's machines, while Ulla and Irene are in separate cells. Before Jeff can be fatally tortured, Femina falls under attack by a squadron of helicopters, a strike force of Sir Masius. Jeff's guards leave him to help defend Femina, allowing him to escape the machine, and save Irene and Ulla. Before they can go far, their path is blocked by Lesley, now revealed to Jeff as one of Sumuru's minions. Jeff nevertheless convinces her to join his escape, with Ulla and Irene in tow.
As Femina's defenders quickly fall to Sir Masius's helicopters, Masius lands in person, hoping to grab Sumuru's riches personally. However, Sumuru reaches it first, and arms the city's self-destruct, preferring to destroy Femina rather than let Masius or any men have its wealth. Sutton barely escapes Femina before it self-destructs, presumably killing both Sumuru and Masius, and wiping out her army of women.
In the final scene, Sumuru is shown to have survived the fall of Femina, and now leads a troop of black-clad women aboard a ship leaving Rio.
