Solo and Illya try to protect Professor Lillian Stemmler from THRUSH after she invents a drug called Plus X, which heightens all the human senses. Unbeknownst to them, she is a THRUSH agent herself who at first cooperates willingly, then has second thoughts. But THRUSH agent Rollo kidnaps her daughter Leslie and forces her to cooperate. The drug is given to three THRUSH agents who will attack a U.S. government plutonium plant using their superior senses, while at the same time the guards will be given a dose of Minus X, which dulls the senses.

Solo investigates a girl's school for the daughters of VIP's, including Miki Matsu, who has valuable secret information from her father, a Japanese diplomat. THRUSH agent Jason Sutro has gained the cooperation of the headmistress, Hester Partridge. The assistant dean, Verity Burgoyne, and all of the girl students have been brainwashed by Sutro to go into a trance upon hearing a recording of Brahm's Lullaby, and are ordered to kill Solo.

Frank Cariago, the U.S. head of a crime syndicate, is under pressure from Uncle Giuliano. Cariago decides to buy a movie production directed by Sheldon Veblan so his girlfriend, Ginger Laveer can have the starring role. But the picture is a disguised plan to drop a bomb on the family's biggest rival-Las Vegas.

Solo goes to Sweden to obtain a new suspended animation device from Dr. A.C. Nilson, but the device and its inventor are taken by THRUSH. The doctor's daughter Neila helps Solo find him, and the doctor's assistant, Inga Anderson also feigns cooperation but is actually working for THRUSH agent Heinrich Beckmann. Beckmann uses the device to invade U.N.C.L.E.'s Scandinavian headquarters, and only Solo has a chance to stop him.

Solo and Illya visit the laboratory of Dr. True, who has discovered how to extract gold from seawater. But Dr. True dies from a poison given him by THRUSH agent Randolph, and Randolph also kills his wife Amanda. The gold extraction formula was distributed in portions by True to his daughters, in inscriptions on a photo of himself. Solo and Illya meet Sandy True, who accompanies them to Rome, to find her half sister Margo, now unhappily married to the destitute Baron de Fanzini to London, to find the next sister Imogen, who has been arrested by a Constablefor indecent exposure and finally the Alps, to find Yvonne, who is breaking off an unhappy relationship with Karl Von Kesser. But after decoding the message in Japan, Solo, Illya and Sandy are captured by Randolph and taken to THRUSH central for execution.

Solo and Illya visit the laboratory of Dr. True, who has discovered how to extract gold from seawater. But Dr. True dies from a poison given him by THRUSH agent Randolph, and Randolph also kills his wife Amanda. The gold extraction formula was distributed in portions by True to his daughters, in inscriptions on a photo of himself. Solo and Illya meet Sandy True, who accompanies them to Rome, to find her half sister Margo, now unhappily married to the destitute Baron de Fanzini to London, to find the next sister Imogen, who has been arrested by a Constablefor indecent exposure and finally the Alps, to find Yvonne, who is breaking off an unhappy relationship with Karl Von Kesser. But after decoding the message in Japan, Solo, Illya and Sandy are captured by Randolph and taken to THRUSH central for execution.

Solo and Illya go to visit a Nazi war criminal in prison, Von Etske (Will Kulova), but find that he has escaped with the aid of a special thought controlling device. His escape was observed by photographer Terry Cook (Judy Carne). Von Erske is joining forces with Zorgon Gurnius (George Macready), a fellow former Nazi, to reactivate their wartime triumvirate. Illya intercept Nexor, and finds that he is his exact double. He decides to impersonate Nexor to foil the plan.