At the Del Floria tailor shop in New York, a man comes in to leave his jacket off and casually tosses his other jacket over a nearby TV. He then gasses Del Floria unconscious with a trick cigarette lighter and lets three armed men in. While the first man puts on an apron and impersonates Floria, the other three men open up a secret door in the back of the changing room and enter a high-tech reception room. They gas the receptionist unconscious before she can sound the alarm, put on badges, and two of them go on into the next hallway.
As they go past technicians and agents, the alarms go off and the intruders run down the hall. One gets cut off and the other one uses thermite to burn through a door. On the other side he finds a man waiting for him. The intruder fires and discovers that there's a bulletproof glass between them. The newcomer slips around the glass and shots down the intruder before he can get another shot off.
Act I: An Assault Force from Thrush
The man, American agent Napoleon Solo of U.N.C.L.E., checks the dead man as his partner, Russian Illya Kuryakin, arrives. Illya confirms that the man used thermite and that there is no identification on the man's body. They figure that THRUSH sent the intruders to kill their leader, Alexander Waverly. Waverly arrives and confirms their speculation, explaining that they intercepted a THRUSH communique involving Andrew Vulcan, the head of THRUSH's eastern seaboard operations. Vulcan owns the United Global Chemical Corporation, and the communique reveals that he plans to assassinate Premiere Ashumen, the head of the newly independent Western Natumba. They can't move on Vulcan due to lack of evidence, so Waverly orders Napoleon to go to Washington, meet with Ashumen, and prevent his assassination.
The head Channel E briefs Napoleon on Ashumen and his two closest allies, revolutionaries General Molte Nobuk and Economics Minister Jean Francois Soumarin. Vulcan is a recluse who spends most of his time either at his estate or at his plant in Maryland, and keeps a private army. As Napoleon heads out, Illya arrives and gives him Vulcan's 1949 high school yearbook. It turns out that the industrialist had a girlfriend, Elaine May Bender, of Middleton, NY.
In Middleton, a nervous Reverend Anderson visits Elaine and tells her that his bishop wanted him to come over. Napoleon arrives and Anderson reluctantly vouches for him on his superiors orders. Once he leaves, Elaine's children go off to school and Napoleon explains that he wants her to take a flight with him to Washington. He explains that he's with U.N.C.L.E., which opposes the ruthless THRUSH, and they need Elaine to help Napoleon get close to Vulcan so that he can save Ashumen's life. Napoleon insists that she can't tell her husband about why they need her, even though they've applied influence to make sure he agrees to her sudden trip. Elaine refuses to even consider the idea of leaving her family.
Shortly, Elaine is with Napoleon on a flight to Washington. He flirts with an attractive female passenger and then sits down with Elaine, and she wonders how she was appointed a special delegate to a parent's conference in Washington. Napoleon tells her that the original delegate is conveniently "ill", and tells Elaine that she will be posing as Elaine Van Avery, the wife of a deceased oil magnate from Oklahoma. As Napoleon settles down for a nap, the woman he was flirting with carefully watches Elaine in her compact mirror and checks the gun in her purse.
Act II: Trouble on the Potomac
When they arrive at the hotel, Napoleon gets Elaine settled in and comes in the next morning with racks of expensive clothing. He explains that she will be attending Vulcan's party at his estate that night and she's on the guest list, as is Ashumen. He reminds her that she's playing the part of a widow and warns her that THRUSH will kill them both if she slips up in public. Napoleon says that Ashumen will be inspecting Vulcan's plant the next morning, and he wants Elaine to let Vulcan approach her. Meanwhile, he'll be at the party as a guest. If Elaine learns anything suspicious, she's to single him. Napoleon leaves her to get ready and Elaine admires the expensive clothing.
That night, Elaine arrives at Vulcan's estate and the hostess, Gracie Ladovan, shows her around and introduces her to Ashumen's delegation. Vulcan spots Elaine and comes over to greet her, and she tells him her cover story. He gets Elaine out on the balcony and admits that he hasn't been involved with a woman in many years. She suggests that they take it slow and notices Napoleon inside, talking to Ashumen. Vulcan recognizes Napoleon and dismisses him as a party crasher. As they talk, Vulcan's chief engineer, Alfred Ghist, comes in and points out Napoleon, calling him "their friend's uncle." He tells Vulcan that Napoleon won't be interfering with them and his superior tells him to make sure that Napoleon doesn't talk to the wrong person.
Once Ghist leaves, Vulcan explains to Elaine that he's building a plant for Ashumen similar to the one he has in Maryland. Elaine invites him to dance and Vulcan eagerly accepts. She signals Napoleon, who cuts in on Vulcan and they exchange whispers. Elaine tells him what she's learned and they notice that Ghist is talking to Ashumen's people. Napoleon slips a small bottle to Elaine and tells her to put a few drops into Ashumen's drink, assuring her that it will save his life.
A short time later, Ashumen is bent over in pain and his people take him up to his room. Elaine, realizing what she's done, glares at Napoleon and then runs off after Ashumen. Napoleon goes to get his car and Ghist tells the valet that he's rigged a gas grenade to the steering wheel. When the car reaches 50 miles per hour, it will go off. Napoleon drives back and calls in to report that Ashumen won't be attending the tour the next day. He speeds off and the grenade goes off, knocking him unconscious as his car skids out of control into the Potomac.
Act III: Incident in a Hotel Suite
Vulcan takes Elaine back to her hotel room and they kiss. She suggests that they go out to a night club and asks for a half hour to get ready. Once Vulcan steps outside, Napoleon staggers in from the bedroom and collapses. Elaine tends to his head wound and complains about what he had her to do to Ashumen. Napoleon insists that the drops will have no permanent effect, but Elaine warns him that the tour is continuing as planned even though Ashumen won't be attending. Napoleon realizes that there's something bigger going on and tells Elaine that he'll have to investigate it immediately. He orders Elaine to somehow convince Vulcan to take her on an immediate tour of his plant. She refuses, saying that isn't convinced that Vulcan is a bad guy and that she still has some feelings for him. Napoleon reminds her that Vulcan tried to have him killed and wonders if she's afraid of losing the glamour and glitz, and having to go back to her life as a housewife.
As Elaine considers what Napoleon is saying, Vulcan knocks at the door and Napoleon tells her to stall the industrialist for fifteen minutes. She does so and goes out into the hallway with Vulcan, and Napoleon calls the airport to charter a plane.
Vulcan readily takes Elaine to his plant, happy that she's interested in what he does. As they sit down to catch their breath, a plane falls overhead and someone bails out of it. Vulcan tells his guards to move in and sends Elaine inside, but she ignores him and slips away. Vulcan then goes to the plant's main reactor room and tells Ghist that they have an intruder, and orders him to seal off the reactor. He then joins his men, who have found the "intruder." It's a dummy, and Vulcan realizes that it's a diversion staged to force him to go to the reactor room.
Napoleon slips into the reactor room and starts to look around. He's forced to hide when Vulcan and his men arrive, and slips out past the guard. The guards give chase and Napoleon manages to make his way outside. He goes up the stairway of a tower, only to find the door at the top locked. As the guards open fire, Elaine opens the door from the inside. They make a run for it and get to a parked car. However, when they get in, they find an armed Ashumen waiting for them. He apologizes for not being able to rise to greet them because of the drug, but clearly knows that Napoleon drugged him. Elaine warns him that Vulcan plans to kill him, but Ashumen tells her that he knows all about it... and that it's Nobuk and Soumarin that have to die.
Act IV: The Assassination
Vulcan and his men take Napoleon and Elaine to a room beneath the reactor and hang them up from a pipe. Vulcan tells them that the steam will take almost two hours to fill the room and they will die a slow death. Napoleon, having worked out THRUSH's scheme, congratulates Ashumen on setting up the entire thing so that THRUSH can take over his country. Vulcan admits that once THRUSH has its own nation to control, they will have a manufacturing base and a diplomatic immunity to further their goals. Since Soumarin and Nobuk would object, THRUSH will eliminate them by blowing up the reactor during the tour. Ashumen will conveniently escape and return to hand his country over to THRUSH. Vulcan tells Ghist to remove Napoleon and Elaine's bodies in the morning and plant them so that they are discovered after the explosion. Ashumen apologizes to Elaine for her being accidentally swept up in the assassination and leaves, and Vulcan opens the steam valve and walks out with his men.
Once they're alone, Napoleon swings his legs up so he grabs the water pipe, then inches over to the nearby joint and starts kicking at it. It finally gives and he turns off the steam, and then slides Elaine's chains off of the pipe. They collapse from exhaustion and Elaine starts crying about how her family only sees her as a housewife. Napoleon assures her that she was the most beautiful woman at the party and hands her a discarded earring, and they both burst into laughter.
The next morning, Vulcan takes the delegation on the plant tour. Ghist goes to check the prisoners and Napoleon knocks him out from behind when he enters the room. He then sends Elaine outside and goes to the reactor room. Spotting the gauge inching into the red, Napoleon yells at the Soumarin and Nobuk to get out. They run for it with the rest of their delegation, and Napoleon follows them as Vulcan and Ashumen are caught in the explosion. Unaware of Ashumen's treachery, the two ministers assume that he died nobly, while Elaine finds Elaine and leads her away.
Later, Napoleon and Elaine return to Middleton and she assures the agent that she's glad to be going home. Napoleon has bought presents for her family and she thanks him with a kiss on the cheek. She goes to the terminal where her happy family is waiting for her. Napoleon watches them with satisfaction and then sits down to sleep for an hour before the plane continues to New York. The attractive stewardess comes over to ask if there's anything she can do to make him comfortable, and Napoleon clearly has a few ideas.