Thrush is conducting an operation using a mortuary as a front. One Thrushman appears to commit suicide. Another is falling for the "dead" man's girlfriend. UNCLE is using the girlfriend, whose name is Jojo Tyler and the "dippy blonde" of the episode title, to break the operation.
Thrush has a mole inside UNCLE's New York headquarters. All signs point to George Dennell, a good-natured UNCLE employee who envies enforcement agents such as Solo and Kuryakin. Dennell protests he is innocent and Solo takes a big chance on George.
A British politician's career is suddenly rejuvenated after he's been exposed to a device that reverses the aging process. But he's also a prisoner. He needs additional treatments and the machine is now under the control of Thrush and the politician's manipulative wife.
Kuryakin, who has photographed Thrush codes needed by UNCLE, is captured by the criminal organization. He parachutes, with a stewardess, from a chartered aircraft over the North African desert. They end up at a Foreign Legion post, still manned by Basil Calhoun, who doesn't know the legion has been disbanded. Both Solo and Thrush are trying to get to Kuryakin.
After Solo and Kuryakin are sidelined, Waverly turns to two agents: April Dancer, a young woman, and Mark Slate, a veteran operative who, it turns out, is older than 40, the retirement age for enforcement agents.
The title refers to criminal Rafael Delgado, an expert diamond thief. He's involved with a band of criminals, who dress in British style (bowler hats and umbrellas) but who speak Italian. It's implied they are Mafia types trying to pass themselves off as British. Delgado, even while in prison, has pulled off a stunning robbery with the help of the gang. So many diamonds are missing it could affect the world's economy.
Thrush mistakes Buzz Conway for a scientist. Solo and Kuryakin use that to their advantage while trying to smash a Thrush operation headed by Marvin Elom. Narcissus Darling, a Thrush femme fatale, also is involved.
Count Zark, a Thrush agent, operates out of Transylvania and has developed a worldwide menace involving bats. Meanwhile, UNCLE is checking out a young woman who seems capable of reading the minds of others. Solo and Kuryakin discover how both events are related.
Professor Lillian Stemmler is collaborating with Thrush. She has developed two formulas: Plus X, which heightens the senses, and Minus X, which temporarily transforms people into child-like morons. Thrush intends to use both to steal U.S. military secrets.
Thrushman L.C. Carson has kidnapped Chief Highcloud of the Cardiac tribe of Indians in Oklahoma. The criminal organization also is menacing the chief's daughter, Charisma, who dances in New York. It turns out Thrush is working on a hydrogen bomb on the Cardiac reservation. Solo leads a group of Indians, all riding motorcycles, in a climatic fight with the Thrush forces.
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 (Robert Vaughn) and Kuryakin (David McCallum) 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 (Noel Harrison). Slate is to prepare a stripper to take the place of a woman in the employ of Thrush (Joan Collins in a dual role).
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.