THRUSH is so eager to get their hands on a boy genius that they murder the boy's father. But then they have to deal with the boy's madcap aunt.

When secret ""File 40"" documents turn up outside headquarters, Waverly assigns Solo and Illya to find out if George Donnell is a double agent. Carla Drosten is too anxious to accuse Donnell and Solo must use special ""Waverly rings"" to try and expose the real double agent.

Illya parachutes out of a plane carrying THRUSH code documents, but lands in the desert at an abandoned Foreign Legion post run by Captain Basil Calhoun (Howard Da Silva). Illya is accompanied by a stewardess, Barbara (Danielle DeMetz) while Solo races to find them before THRUSH does.

Solo and Illya must protect Premier Georgi Koz, a Khrushchev like European leader, on a visit to New York, Priscilla Worth is befriended by Loz and she takes him to a school for Santa Clauses run by Francis X. O'Reilly where just one of a series of assassination attempts against him must be thwarted by the two U.N.C.L.E. agents.

President Nasasos Tunik visits Paris. His assistant, Malanez, is determined to persuade the president that the French are his enemy, and arranges for various embarrassing and insulting incidents to occur. Solo and Illya are assigned to see that the visit goes smoothly, but Tunik falls in love with Candyce, and Malanez plans to disgrace Tunik by framing him in a plot to steal the body of Napoleon from his tomb.

Jacqueline Midcult writes a best selling novel, The Pieces of Fate, which U.N.C.L.E. recognizes as being based on a series of missing THRUSH diaries. She loses her memory during a THRUSH attempt to kill her, and THRUSH agents Ellipsis Zark and Jody Moore, a book critic, plot to kidnap her and find out where she found the diaries. Solo and Illya take Jacqueline to a small town where her Uncle Charly and Aunt Jessie live, to try and revive her memory, but Zark and Moore follow and they all converge on the attic where the diaries are hidden at the same time.

In Portugal, Solo is taken prisoner by Pharos Mandor (Jack Lord), a THRUSH chief ready to defect if his arch rival, THRUSH assassin Stepan Valandros (Nehemiah Persoff), does not kill him first. Solo meets Mandor's girlfriend, Cathy Welling (Leslie Parrish). Mandor tells Solo U.N.C.L.E. must kill Valandros or he will not defect, but his real aim is to eliminate Valandros and promote himself up the THRUSH ranks.

Oliver Barnes (Lawrence Mantaigne), a THRUSH agent, unsuccessfully tries to destroy a package. Solo and Illya suspect a connection between this and a new ""molecutronic gun"" developed by Dr. Fabray (William Marshall). Solo runs into Abbe Nelton (Anna Capri). Fabray turns out to be a THRUSH collaborator. The entire scheme was a plan to get Illya to take the ""gun"" actually a bomb, into U.N.C.L.E. headquarters in Trojan Horse fashion.