The unnamed 'Z' gang (so named because of the 'Z' insignia on their helmets) are the main antagonists in the Thunderbirds episode 'The Uninvited'. They are portrayed as being a Middle Eastern terrorist group, operating from a base underneath the Lost Pyramid of Khmandides in Egypt.
Appearance in the show
The 'Z' gang only appear once in the beloved 1960's superhero series; they appear in the 10th episode of season 1, 'The Uninvited'.
Three 'Z' fighters are shown to open fire on Thunderbird 1, causing it to crash in the middle of the Sahara Desert, injuring Scott Tracy. After Scott's younger brother Virgil, along with Brains and Tin-Tin, arrive in Thunderbird 2 to aid Scott and repair Thunderbird 1, they talk to the people who rescued Scott until Virgil et al arrived. It's revealed that the two desert truckers are explorers, looking for the lost Pyramid of Khmandides. Despite some hardships (the trailer of their truck overturns and blows up, taking their water, fuel and the best part of their radio with it), the two explorers make it.
The explorers use the majority of their remaining 50 gallons of fuel to go to a now-dried-up waterhole, and start radioing for help. With three of their seven transmitting components buggered, their radio signals don't get to their basecamp, but they do get intercepted by Thunderbird 5. Alan then informs Jeff, who immediately dispatches Scott in Thunderbird 1 to Egypt to observe them (despite Scott previously taking Alan up in Thunderbird 3 for his monthly changeover with John).
They were at war with another tribe called the 'Nebab' and were responsible for shooting down Thunderbird 1, so they could copy its design and use it to make fighter jets to defeat the "Nebab".