A school on Long Island has a study body made up of teenagers of scientists and other prominent people. Thrush is secretly running the facility and programming the students to steal secrets from their parents. Kuryakin acts as bodyguard for the teenage daughter of a scientist while Solo gets inside the school.
Solo is attacked by a cyborg (a mostly electronic being with some human organs) while on a mission. Later, it's discovered that a Thrush front company is seeking a $1 billion loan. Eventually, Solo and Kuryakin uncover a plot to mass produce cyborgs.
Solo and Kuryakin narrowly escape an attempt on their lives in Venice. Solo has caught pneumonia as a result of a swim in the Venice canals. As a result, he is replaced on the mission by Mark Slate. Slate is to prepare a stripped to take the place of a woman in the employ of Thrush.
Munitions magnate Brutus Thor intends to assassinate a Ghandi-like figure who protests for peace. Solo and Kuryakin must stop a plan to blow up a disarmament conference where Thor's target will deliver a major speech.
Thrush, led by the nervous Louis Strago and his assassin secretary Miss Diketon, has embarked on an ambitious project. The organization has recruited a Nazi scientist who once theorized by changing the Gulf Stream, you could make Iceland into a lush, green country and make Europe into a frozen wasteland. Thrush is prepared to make the scientist's dream a reality. By the end of Part I, Napoleon Solo faces a forced marriage to a relative of retired gangsters while Illya Kuryakin is being tortured by Miss Diketon.
Napoleon Solo sets off for a Thrush island, from where the criminal organization will fire nuclear missiles to alter the path of the Gulf Stream. During the course of events, Solo enters into an uneasy alliance with retired gangsters; Miss Diketon betrays Thrush leader Louis Strago; and Illya Kuryakin nearly drowns.
Kuryakin is captured while investigating a mysterious threat in an African country. Solo is sent to pick up his trail. The agent encounters a woman simply named Girl who has a pet gorilla named Baby. For reasons not readily apparent, Solo at one point dances with the gorilla.
Scientist Dr. Adrian Cool informs UNCLE that his radio telescope is showing a spaceship heading to Earth. At the same time, Dr. Cool's daughter is kidnapped. All of this has been devised by evil industrialist Simon Sparrow as a means to grab power for himself.
Tunick, the leader of a former French colony, is the subject of a plot led by his No. 2 official. That official wants to manipulate Tunick into stealing Napoleon's tomb, a move that will embarrass the leader and drive him from office.
Thrush steals a powerful UNCLE-developed explosive hidden in a hula doll. The explosive will detonate if heated to 90 degrees Fahrenheit. This is bad news because New York is in the midst of a heat wave. Solo and Kuryakin must also contend with a pair of brothers, each vying for power within a New York Thrush operation.
A young woman's novel reads uncomfortably similar to UNCLE cases involving Thrush. UNCLE investigates whether the woman, who has lost her memory following an attack, may have had access to secret Thrush records.
A scientist, Dr. True, has developed a process for extracting gold from seawater. He dies just as he is performing a demonstration for Solo and Kuryakin. His death has been arranged by Thrush. However, Dr. True has scattered his formula to "the four winds" -- stepdaughters he has around the world. Both UNCLE and Thrush pursue the formula. As part I ends, Solo, Kuryakin and Sandy, Dr. True's natural daughter, are left tied up in an UNCLE plane by Thrush operatives. The Thrushman parachute to safety as the UNCLE plane flies on with no one at the controls.
The UNCLE agents free themselves and get control of the UNCLE plane just before it runs out of fuel. Solo and Kuryakin, accompanied by Sandy True, resume their search for the late Dr. True's formula. They are dogged by Randolph, a Thrushman, accompanied by his "karate killers." They all eventually wind up at a secret Thrush installation where the criminal organization plans to use Dr. True's process to produce gold on a massive scale.