Prior to an important conference at U.N.C.L.E. headquarters, a bizarre series of events occurs revolving around a strange man Mr. Hemmingway (Richard Haydn) who keeps appearing and disappearing at will in the building. Solo and Illya learn that he has been placed there by Waverly to test the security system, but THRUSH also has an inside agent, Riley (Peter Haskell), who plants an exploding false tabletop on the conference table at the direction of Dr. Egret (Lee Meriwether). Solo has only a few minutes, before the conference is to begin, to try and find out who the infiltrator is.

Solo and Illya devise a clever scheme to discredit a Balkan intelligence chief (Werner Klemperer) with a bogus secret gas. With the help of a bankrupt exterminator (William Shatner) and his wife, Illya poses as a fellow countryman and exploits the paranoia of the chief and his bumbling assistant (Leonard Nimoy).

U.N.C.L.E. must transport captured THRUSH official Egon Stryker (Ralph Taeger) from New York to Washington with THRUSH rescuing him. While Waverly takes a decoy along one route, Solo and Illya take the real Stryker. Or is it ? But in a mixup, Fran Parsons (Joanna Moore),a secretary on her lunch hour, gets handcuffed to Stryker and has to go along.

U.N.C.L.E. Portuguese translator Mandy Stevenson (Barbara Feldon) hungers for more excitement, so Solo sends her on a fake mission to get Waverly's tobacco not realizing she has taken an important microfilm that THRUSH is after. Solo, Illya and THRUSH pursue her through the streets of Manhattan, and eventually she and Solo are captured by the THRUSH leader, Victor Gervais (Cesar Romero), and Solo has to use his skills as a marksman to escape.

Solo gets an unwelcome young companion (Kurt Russell), a ten year old who wants Solo to marry his widowed mother, in his efforts to find the source of a deadly chemical that killed an entire Scottish village. In a race to find the source with General Yokura (Leonard Strong), Solo eventually uses a ring on the finger of a statue to pinpoint a cave where the deadly chemical has leaked from, but has to use his wits to escape when he is trapped there by Yokura and his men.

When a minor European leader is assassinated, Solo steals a medallion from his body containing a microdot listing the THRUSH agents in the country, but must elude the secret police and outwit intelligence agent Satine (Ricardo Montalban), who also wants the medal. Teacher and tour guide Sara Taub (June Lockhart) and her high school students are used by Solo to try and sneak the medal over the border.

Waverly's cousin is killed by his own dog when he refuses to sell his stock to a gypsy named Delgrovia (Paul Lambert), so Illya investigates and, with the help of Ursula (Susan Oliver) and dog expert Guido Panzini (Pat Harrington Jr.), traces two of the dogs to Delgrovia's estate, where he and Ursula are soon cornered by a pack of deadly Doberman pinschers.

Waverly is poisoned, and taken to a hospital where he is subjected to the rays of a brain-altering machine by THRUSH agent Dr, Dabree (Elsa Lanchester) designed to make him ineffective without killing him. While investigating others who have suffered the same fate, Solo finds Cecille Bergstrom (Yvonne Craig) and together they try to unravel the mystery and save Waverly.

Squire G. Emory Partridge (George Sanders) kidnaps Illya and lures Solo to his manor in order to kill them off in his dungeon torture chamber in revenge for a past encounter. But Peggy Durance (Bonnie Franklin) helps them escape, with the unintended aid of Partridge's bumbling wife Edith (Jeanette Nolan).

Retired U.N.C.L.E. agent Albert Sully (Martin Balsam) is brought back to impersonate a crime syndicate leader, and insists on leaving Solo and Illya behind and running the operation himself. He reunites with wartime fellow agent and old flame Bryn Watson (Barbara Shelly) and tracks down a secret crime conference, with the exasperated Solo and Illya trying to keep him from being killed in the process.

The conuation of "The Quadripartite Affair". Gervaise Ravel (Anne Francis) returns and Solo and Illya again enlist Marion Raven (Jill Ireland) to help them pursue her through the Andes, where the treachery of police lieutenant Manuera (James Frawley) impedes their efforts to capture Ravel.

Solo and Illya travel to the Riveria, where they search for a serum that supposedly provides great strength and even brings the dead back to life. There they meet Madame Streigau (Marian Moses), who is actually Dr. Egret of THRUSH, as well as Lucia Nazarone (Danica d'Hondt) and her bevy of beautiful blonde helpers. With the help of teacher Lavina Brown (Kipp Hamilton), they trick Mazarone into thinking they have the serum also, and become the targets of her "superwomen".

When a rare postage stamp at an auction reveals that a former SS scientist, Professor Amadeus, is still alive and experimenting with the secret of "suspended animation," Solo and Illya enlist the aid of college students Terry Brent (Brook Bundy) and Chuck Boskirk (Burt Brinckerhoff) to beat THRUSH agent Angelique (Janine Gray) to Amadeus's lab, where Solo is captured so his blood can be used to revive the suspended body of the Fuhrer.

On vacation Solo is asked by an old love interest, Clara Valdar (Madlyn Rhue) to help her smuggle Gypsy leader Emil (Jacques Aubuchon) out of Terbuf with evidence that the corrupt head of the secret police, Colonel Morisco (Alan Caillou), has been embezzling foreign aid money with the aid of the repulsive Major Vicek (Albert Paulsen).

THRUSH agent Anton Korbel (Ken Murray) guards THRUSH's fifty-million dollar treasury in a vault beneath his casino, and Solo and Illya recruit a naive girl from Minneapolis, Susan Callaway (Marlyn Mason), and shady safecracker Marcel Rudoph (Dan O'Herlihy) to break in and destroy the cash.

THRUSH creates a double for Solo, and with the seductive aid of Serena (Senta Berger), Darius Two (Michael Evans) kidnaps Solo and substitutes the phony into U.N.C.L.E.'s efforts to transport the code to a secret new weapon. Illya and stewardess Sandy Wister (Sharon Farrell) eventually realize a switch has been made, and the real Solo escapes and in a climactic scene battles "himself".

Solo and Illya protect a young Himalayan prince (Michel Petit) and his nurse (Susan Seaforth) from THRUSH agents who want to kidnap him one of whom turns out to be his bodyguard.

Solo is aided by tourist Bernie Oren (Glenn Corbett) in finding a black market auction in Hong Kong run by Mr. Cleveland (Gavin McLeod) that sells military secrets, including a microfilm in a rare coin. Oren is infatuated with Heavenly Cortello (Karen Sharpe), and hinders more than he helps Solo, while Illya tries to infiltrate the auction disguised as a Mongolian warlord.

After a THRUSH attempt to kill Waverly is thwarted, Solo is assigned to prevent the assassination of a visiting African premier (William Marshall) at the hands of Andrew Vulcan (Fritz Weaver), and with the help of a housewife (Patricia Crowley) he learns that the premier himself is allied with THRUSH and plans to kill his two top aides (Ivan Dixon and Rupert Crosse) in a fake accident.

Solo investigates the curious death of an air force man in Iowa with scuba gear, and with the help of Jill Denison (Katherine Crawford) uncovers a plan by Clint Spinner (Slim Pickens) to steal a missile- plane from a secret base under a farm.

In Yugoslavia, Solo and Illya are aided by Marion Raven (Jill Ireland) in stopping Gervaise Ravel (Anne Francis) and her partners from using a fear gas to overthrow various governments. While two of them are captured, two of them escape, and the story continues in "The Giuoco Piano Affair"

In the Yucatan, THRUSH agent Walter Brach (Carroll O'Connor) brainwashes important people from many nations to be "time bombs" who will return to their jobs and do THRUSH's bidding. After eluding Brach's henchmen Chuke (Shuji J. Nozawa) and his leopards, Solo and housewife Chris Linnel (Joan O'Brien) enlist the aid of Mrs. Karda (Dovima), Brach's numerologist, to help them escape.

Solo and Illya investigate a series of kidnappings that lead to a modern-day pirate ship run by Captain Shark (Robert Culp), who is filling his Noah's Ark with craftsmen from all walks of life to repopulate the world after the nuclear holocaust he feels is imminent. With the help of Harry Barnman (Herbert Anderson) and his wife Elsa (Sue Anne Langdon). Solo and Illya pose as shipwrecked sailors to in turn wreck his plans.

Solo attends a revival meeting conducted by evangelist Brother Love (Eddie Albert), who is actually a THRUSH leader constructing a nuclear spaceship with the aid of a kidnapped scientist, Dr. Hradny (Robert H. Harris). College student Pearl Rolfe (Maggie Pierce) is kidnapped by Love's followers, and Solo must rescue both victims and destroy Love's plan at the same time.

Solo goes to India to investigate the death of a fellow U.N.C.L.E. agent in an airline crash, and discovers that a cult of Thuggees headed by a maharajah (Murray Matheson) is causing the crashes in order to loot the passengers. But Solo and THRUSH agent Tom Simpson (Linden Chiles) are looking to recover one item in particular - U.N.C.L.E.'s new polygraph device.