Solo uses the former girlfriend (Kathryn Hays) of 2 brothers, Joseph and Max Van Schreetan (Gerald Mohr, Lloyd Bochner), to thwart their plan to control the diamond market, while at the same time THRUSH agent Corio (Alfred Ryder) tries to steal their cache of gems.

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).

U.N.C.L.E. and Thrush agents compete to gain possession of a mind-reading device created by a magician who has been murdered by Thrush.

Squire G. Emory Partridge returns, having acquired in Alaska a large quantity of Quadrillenium X, a very heavy metal with high magnetic powers which THRUSH wants to use to disrupt world communications. Solo and Illya are nearly killed by the local Eskimos, but are saved by the chief's daughter, Murphy. Partridge and his niece Victoria capture them, but they escape only to end up in a tavern brawl.

Solo uses hillbilly clairvoyant Clemency McGill to counter the efforts of Count Zark, a vampirish THRUSH agent who plans to use radioactive bats to jam the world's radar systems from his castle. Solo and Clemency make their way to the castle with Illya, but Zark manages to release the bats.

THRUSH agent L.C. Carson uses an Indian reservation as a front for his plan to assemble a hydrogen bomb, and kidnaps the tribe's chief, High Cloud, to ensure cooperation. Solo arranges for the Chief's daughter Charisma to return, but she ends up being kidnapped along with Solo and Illya, who is disguised as an Indian. Solo and Illya, along with a band of young Indian warriors on Motorcycles, encircle Carson and his men and try to stop them before its too late.

Solo and Illya are asked by Adam Tenza (Chad Everett) to protect his millionaire industrialist father Mark Tenza (Broderick Crawford) from assassination by THRUSH, much to the resentment of the elder Tenza. When Tenza is killed by a bomb, Solo and Illya must find out where J Tenza's other son, James is, to protect him also. But Adam Tenza is actually planning to kill J, in order to hand his father's empire over to THRUSH.

Illya recuperating in the hospital, is kidnapped by an old enemy, Viktor Karmak (Darrin McGavin), to lure Solo into a trap. Solo follows a clue to a twelve block condemned area of Manhattan, and finds modern artist Sheila Van Tillson (Marlyn Mason). Karmak appears and announces that Solo has until dawn to find Illya before a deadly gas kills him, while Karmak and his pet jaguar will try to hunt Solo down at the same time.