In his London, England apartment, Ken Pile tends to his aquarium, addressing his favorite fish as “Wanda.” He is interrupted when an American woman named Wanda Gershwitz arrives with her “brother,” Otto, a Friedrich Nietzsche enthusiast who teases Ken about his stutter and love of animals. When Wanda’s boyfriend, George Thomason, joins them, Otto demonstrates his prowess as a weapons specialist, hoping to join George, Wanda, and Ken in a jewel heist. Otto appears jealous as George kisses Wanda, and although he loudly calls her “Sis” upon leaving, he warns Wanda not to let George touch her. The next day, Otto, George, and Ken steal $20 million worth of diamonds while Wanda, disguised as a male chauffeur, waits in a limousine. The foursome speeds away from the crime scene, and Wanda nearly runs over an elderly woman, Eileen Coady, as she walks her three dogs. Ditching the getaway car, Otto and Wanda follow George to a garage, where he stashes the loot in a safe. Later, Wanda kisses Otto, who is secretly her boyfriend. They betray George by reporting his name to the police, who apprehend him moments after he hides a key in Ken’s fish food container. Wanda and Otto return to George’s safe for the diamonds but find it empty. They visit George in jail, where George and Wanda rehearse their shared alibi. George suggests he could reduce his sentence by revealing the whereabouts of the jewels and naming his accomplices. Outside the jail, Wanda recognizes George’s attorney, Archie Leach, and introduces herself as a legal student and fan of his work. Befuddled by her flirting, Archie drives away leaving his briefcase on top of the car, and Wanda tells Otto of her plans to seduce Archie in case George reveals the location of the diamonds. Ken locates the fish food container with George’s key and hides it in a treasure chest inside his aquarium, as instructed by George. However, Wanda spies on him, then steals the key and places it in her necklace locket when Ken is distracted. Eileen Coady, the elderly woman nearly killed by Wanda, identifies George in a line-up. Wanda goes to Archie Leach’s office to interview him about George’s case and learns that George plans to plead “not guilty” despite the old woman’s claims. Upon learning that Wanda knows George, Archie refuses to speak further, but she claims not to care, suggesting she came to see him for personal reasons. Archie returns home to his passionless wife, Wendy, who complains incessantly about her upper class life and their spoiled daughter, Portia. While the married couple retires to separate beds, Otto entices Wanda by speaking Italian while they have sex. The next day, at George’s hearing, Otto sees George pass a note to Ken before the court is dismissed. Outside, Otto steals the note, which contains Eileen Coady’s name and address. He realizes George has instructed Ken to kill the eyewitness, and bets Ken one pound that he does not go through with the murder. Meanwhile, Wanda slips her phone number to Archie, who invites her to his house after Wendy and Portia leave for the opera. Otto follows Wanda to Archie’s house, sneaks inside, and observes Wanda and Archie kissing. Hoping to drive Otto away, Wanda asks Archie to fetch champagne; however, Wendy and Portia return home early, causing Wanda and Otto to scatter. As Archie returns with champagne, he finds Wendy on the couch instead of Wanda. She demands to know whose car is blocking the driveway, and Otto appears, introducing himself as a CIA (Central Intelligence Agency) agent, searching the neighborhood for Soviet spies. Although Wanda scrambles to retrieve her locket, which fell on the floor, Wendy picks it up and assumes it is a gift from her husband, as it is engraved with a “W.” Archie embraces Wendy long enough for Wanda to escape. The next day, Wanda calls to demand her locket back, while Wendy happily fondles the necklace at the breakast table. After one failed attempt, Ken tries to murder Eileen Coady by sending an attack dog after her, but the dog kills one of the woman’s dogs instead. Archie arranges to meet Wanda at a friend’s unoccupied apartment, but she is disappointed when he does not return the locket. As the two kiss in bed and joke about Otto’s stupidity, Otto breaks in and hangs Archie outside a window until he apologizes. In a third attempt to kill Eileen Coady, Ken misses his target but runs over another one of her dogs. Wanda convinces Otto to apologize to Archie, but when Otto goes to Archie’s home, he interrupts a burglary. Meanwhile, unbeknownst to Otto, Archie is robbing his own house in order to retrieve Wanda’s locket. Otto overpowers Archie, ties his wrists, and hits him in the head, finally recognizing him just before Wendy returns home. Otto flees moments before Wendy enters the room. With a heap of stolen jewelry lying beside him, Archie sucks the locket into his mouth, then sneaks it into his pocket after Wendy unties his wrists. He explains that a thief overpowered him but refuses to stay for a police report, insisting he has an urgent appointment. At his friend’s apartment, Archie returns the locket to Wanda. She becomes aroused when Archie reveals that he speaks Italian as well as Russian, but she is disappointed to learn that Archie is not rich. The two undress in separate rooms, but Archie is caught, fully naked, when a family who recently rented the apartment arrives. Later, Archie ends the affair with Wanda over the phone. Otto returns to his house to apologize once again, and Wendy overhears as Otto mentions Archie’s fake robbery and affair with Wanda. In a final attempt to kill Eileen Coady, Ken shoots a large block of concrete that hangs outside her house, but it falls on her last remaining dog. Ken leaves his perch in defeat, but discovers on the street that his intended victim has just died from a heart attack. He reports the good news to George, who instructs Ken to buy four plane tickets to Rio de Janeiro, Brazil. At his apartment, Ken and Wanda celebrate the triumph before she rushes off to act as an alibi witness for George. Otto comes to the apartment, and Ken offhandedly reveals that the loot is stashed near the airport, but refuses to give the exact location. Otto ties Ken up and eats all of the fish in the aquarium except Wanda, Ken’s favorite. As Otto traps the fish in his mouth, Ken confesses the diamonds are in a safety deposit box at the Cathcart Towers Hotel. On the witness stand, Wanda incriminates George by saying he left the apartment on the morning of the crime with a sawed-off shotgun. George attacks her and chaos erupts. Having deduced that Wanda is her husband's lover, Wendy slaps Archie and tells him she wants a divorce. In a holding cell, Archie encourages George to reduce his sentence by confessing and demands to know where the diamonds are. George does not answer himself, but says that Ken knows where they are. Archie sees Wanda trying to hail a taxi and forces her into his car. Although he wants to go to Rio de Janeiro with her, he accuses her of being a liar and admits he has fallen in love. Wanda cuddles up to him and says she has the key to the safety deposit box. At Ken’s apartment, Wanda waits in the car while Archie runs upstairs. Meanwhile, Otto jumps in the car and drives off as Wanda calls out to Archie in distress. Archie and Ken follow on a scooter, racing to the Cathcart Hotel. There, Otto and Wanda retrieve the diamonds and check in for the flight to Rio. Before they board, however, Wanda knocks Otto on the head, takes his ticket, and locks him in a closet. Otto shoots his way out and steals another passenger’s ticket, but Archie appears and steals his gun. Otto challenges Archie to a fight and quickly retrieves the gun, leading Archie to the runway and forcing him to climb inside a barrel of waste. As Otto readies himself to shoot his rival, Ken appears, driving a road roller toward a section of wet cement where Otto has unwittingly stepped. Although initially laughing as Ken approaches in the slow-moving vehicle, Otto realizes his feet are stuck and his gun is out of bullets. Ken rolls over Otto, immersing him in the wet cement, and realizes his stutter is gone as he yells in triumph. Archie runs onto the plane and greets Wanda in Italian. As the happy couple discusses their future, they do not notice Otto outside their window, covered in cement. As the plane takes off, Otto loses his grip and falls away.