The Thunderbirds TV series is supposedly set in the 21st century (which at the time of production was still over thirty years away). The specific time frame remains a contentious topic amongst fans, due to contradictory dates seen on newspapers and calendars in different episodes, ranging from 1964 to 2026 to 2065. As he has stated in a number of interviews (most recently for Fanderson's "FAB" magazine), Gerry Anderson's brief to the writers and designers was simply that the series was set "one hundred years in the future" (i.e. 2065). This intent was carried forward in all of the series' contemporary tie-in merchandise, such as the weekly comic strip in TV Century 21 and the Century 21 Mini-Album "Thunderbird 3", wherein Alan Tracy tells listeners that the year is 2065. The close-up appearance of a 2026 calendar in the episode "Give or Take a Million" was later admitted by production designer Bob Bell to have been an error on the part of the prop-maker. 1993 vintage champagne is discussed in "Alias Mr. Hackenbacker", although this only suggests that events in that episode took place after 1993. The date issue is strongly in favour of the mid 2060s, as in the feature film Thunderbirds Are Go the date is shown to be June 2066, and in Thunderbird 6 it is June 2068. In addition, the Zero X spacecraft from Thunderbirds Are Go subsequently appeared in the opening episode of Anderson's next TV series, Captain Scarlet and the Mysterons, which was set in 2068.

The show depicts the adventures of the Tracy family, which consists of millionaire former astronaut Jeff Tracy (who was one of the first men to land on the Moon - which some fans feel adds weight to the 2026 argument, although it is never stated when man first landed on the Moon in the Thunderbirds 'universe', that event having still been in the future at the time of production) and his five sons: Scott (pilot of Thunderbird 1 and principal rescue co-ordinator), Virgil (pilot of Thunderbird 2), Alan (astronaut in Thunderbird 3), Gordonaquanaut in Thunderbird 4) and John (principal duty astronaut on the space station Thunderbird 5) - each named after a Mercury astronaut - Scott Carpenter,^[1] Virgil Grissom,^[2]Alan Shepard,^[3] Gordon Cooper^[4] and John Glenn,^[5] respectively. (Two of the Mercury Seven, Wally Schirra and Deke Slayton, do not have characters named for them. Slayton did not fly as part of the Mercury program due to being grounded from flight status by a heart condition, although he later flew as docking module pilot on ASTP.) Together with Jeff's elderly mother called Grandma Tracy, the scientific genius and engineer "Brains", the family's manservant Kyrano and his daughter Tin-Tin, the Tracy family live on a remote, uncharted island. (

The location of this island is usually assumed to be somewhere in the Pacific (Tracy Island), but there are many inconsistencies in the plots that point to a possible location in the the Caribbean Sea. In the episode 'The Uninvited', Scott is returning from Tokyo and is shot down over the Sahara desert. If Tracy Island really was in the Pacific, he would simply have flown straight there from Japan (also located in the Pacific). But the fact that he is flying over the Sahara, from Japan, means he must be heading for the Atlantic Ocean and, presumably, the Caribbean. But wherever Tracy Island is located (the Atlantic, the Caribbean or the Gulf of Mexico), Scott is returning home by flying AWAY from the Pacific. They are, in secret, the members of International Rescue (IR), a private and highly advanced emergency response organisation, which covers the globe and even reaches into space, rescuing people with their futuristic vehicles, the Thunderbirds.