U.N.C.L.E. suspects that the U.S. industrialist and tycoon Andrew Vulcan, an officer of WASP (an international criminal organization), plans to kill Prime Minister Ashumen of the newly independent African nation of Western Natumba. Solo is assigned by Mr. Allison, the head of U.N.C.L.E. to thwart the assassination and find out why it was planned. Solo thereafter recruits Elaine May Donaldson, a college girlfriend of Vulcan's and who is now a suburban housewife, to help get information from Vulcan on his plans. Solo's thought is that only a personal connection can obtain the information, and Vulcan has neither wife nor close friends. Elaine is given the cover story of being a wealthy widow and is able to not only get Solo the details of the assassination plot, but drugs Ashumen so he is unable to take the tour of Vulcan’s factory which Solo believes will result in Ashumen’s death. Vulcan’s target, though, turns out to be two of Ashumen’s ministers who do not agree with his plans for having Vulcan set up factories in his country. With Ashumen as Premier and Vulcan running the primary industry there, Western Natumba would become a puppet nation of WASP. After a run-in (and brief romantic tryst) with WASP agent Angela, Solo finds out the truth, is captured along with Elaine, and left to die in what is supposed to look like an industrial accident. Solo and Elaine escape, rescue the ministers, and Ashumen and Vulcan die instead in the “accident” they themselves set up. Elaine is returned to her normal life, which she appreciates all the more after the excitement and danger of an U.N.C.L.E. adventure.

Mr. Waverly (Leo G. Carroll) gives U.N.C.L.E. agents Solo, Kuryakin, Arsene Coria (Fabrizio Mioni) from Italy, and Namana (Bill Gunn) from Liberia their assignment: they are to take a top secret code to a hidden location. After they leave, Waverly sends another agent, Australian Kitt Kittridge (Donald Harron), to follow them without their knowledge. However, THRUSH has gotten wind of the "August Affair", though the villains know little more than the name of the operation. They send Serena (Senta Berger) to entice Solo to her apartment. After each unsuccessfully tries to find out what the other is after, he is gassed into unconsciousness and a THRUSH agent, surgically altered to look and sound like Solo, takes his place. An attempt is made to kill Kuryakin to minimize the chance of the substitution being detected, but it fails.

One Spy Too Many is the 1966 feature length film version of The Man from U.N.C.L.E.'s two-part season two premiere episode "Alexander the Greater Affair". It, as does the television series, stars Robert Vaughn and David McCallum. It is the third such feature film that used as its basis a reedited version of one or more episodes from the series. In this instance, the film took the two-part episode and added in a subplot featuring Yvonne Craig as an U.N.C.L.E. operative carrying on a flirtatious relationship with Napoleon Solo (Robert Vaughn); Craig does not appear in the television episodes. It also added in and substituted scenes that, while not out of place in a 1960’s U.S. spy film, were more explicitly sexual than generally shown on U.S. television at the time. Whereas the earlier U.N.C.L.E. films added material to a single episode to create a feature length movie, "One Spy Too Many" removed certain elements of the two-part episode to allow for the added subplot with Craig and other enhanced scenes within the film’s overall running time. This was the last film culled from the series to be theatrically released in the U.S.

Illya Kuryakin is investigating the theft of cats in the Soho area of London, and Napoleon Solo is on a mission to determine the whereabouts of the famous, suddenly youthful appearing, and now missing 83 year old biologist Benjamin Lancer. Solo contacts Lancer’s daughter Lorelei, a model at the Paris salon of Madame Raine De Sala. De Sala orders her associate Olga and another model Do Do to make sure that Lorelei does not speak to Solo. De Sala herself visits Sir Norman Swickert, a very old statesman she knew and admired as a child, and brings along with her Dr. Gritsky – a colleague of Dr. Lancer. Swickert complains of being too old to have political power anymore, and De Sala reveals her desire for such power as he once had, her resentment of not having it available to her since she is a woman, and her ability to make him a younger man with Gritsky’s help.

Solo and Kuryakin are assigned to infiltrate a THRUSH secret base located in a Sicillian winery. The base is run by Louis Strago, who in conjunction with former Nazi Dr. von Kronen is planning to detonate atomic bombs in the Atlantic Ocean. The bombs will cause the Gulf Stream to divert, wreaking havoc in Europe and the United States and warming Greenland sufficiently for it to become a strategic new home for THRUSH (“THRUSHland”). The agents are split up after an encounter with THRUSH, with Solo having to hide overnight in the house of Pia Monteri. When Pia’s grandmother learns of this, she considers it a disgrace to her family’s reputation (despite Solo’s insistence that nothing inappropriate happened) and insists at the end of a shotgun that Solo marry Pia. Solo manages to escape, but Pia and her grandmother enlist the aid of Pia's uncles to find him and return him for marriage. Her uncles are the Stilleto brothers, Prohibition era gangsters in the U.S. who miss the "good old days". Solo barely escapes the wedding.

U.N.C.L.E. agents Napoleon Solo (Robert Vaughn) and Illya Kuryakin (David McCallum) are being attacked with missiles from small helicopters while on their way to see Dr. Simon True. The attackers work for THRUSH and, though they fail to stop the agents, are the titular “Karate Killers.” Dr. True has discovered a process to extract gold from sea water. Fearing theft, he hid the formula, saying someone would have to “hunt down the four winds” to find it. But during his presentation of the process to U.N.C.L.E., he dies from apparently natural causes (though we later discover is due to Randolph’s doing). True's dying words are that his daughter is the key to finding the formula. As the head of U.N.C.L.E., Alexander Waverly (Leo G. Carroll) informs Solo and Kuryakin that the dead man has four step-daughters and one biological daughter, Sandy True (Kim Darby). The consensus is that Dr. True sent each of his four step-daughters something that, when assembled, will mean something to Sandy and from which the formula can be discovered.

U.N.C.L.E. agents Napoleon Solo, (Robert Vaughn) and Illya Kuryakin (David McCallum) witness the aerial destruction wrought on an African village by Dr. Kharmusi using his newly developed “thermal prism”. U.N.C.L.E. then seeks out notorious safe cracker (and former thermal prism researcher himself) Luther Sebastian to help them obtain the device from Dr. Kharmusi. Sebastian is on an island among a religious group called “The Third Way”, a mystical organization with disciples having – except for Sebastian – platinum white hair and all having a belief that when the “Old Man” in their group finally speaks after twenty years of silence The Third Way will have dominion over the world. Wanted in nearly two dozen countries Sebastian agrees to leave the island and help U.N.C.L.E. in return for world-wide immunity from prosecution.

U.N.C.L.E. agents Napoleon Solo (Robert Vaughn) and Illya Kuryakin (David McCallum) investigate when fellow agent Robert Kingsley (Barry Sullivan) and European general Maximilian Harmon (Leslie Nielsen) disappear. Shortly afterward, five of the world's top scientists are mysteriously abducted. The trail leads to the Himalayas, where Kingsley has set himself up as potential world dictator, hoping to use the combined talents of the scientists to build a device that will spread mind-controlling gas throughout the planet. However, his wife Margitta Kingsley (Eleanor Parker) has different plans for the gas.

The criminal international organization THRUSH steals the bomb H975 and demands $350,000,000, to be delivered within 72 hours by their former antagonist, Napoleon Solo. This forces U.N.C.L.E., the United Network Command for Law and Enforcement, has to reactivate the two top agents of its Section II, Solo and Illya Kuryakin, both of whom had left its ranks 15 years before and are now pursuing other lines of civilian work—Kuryakin as a fashion designer whose resignation was acrimonious and precipitated by a professional disaster, Solo as a marketer of computers and independent businessman. Equipped in their original fashion, Solo and Kuryakin search for the bomb and attempt to close down, permanently. what proves to be a splinter THRUSH group; the original organization had fragmented in 1968 after its failure in the "Seven Intellectual Wonders of the World" affair and has yet to regain the power to threaten worldwide law and order that it had possessed up to that time.