The film is largely set on Skyship One - a futuristic airship designed by Brains, the inventor of International Rescue's Thunderbird machines. The plot sees Alan Tracy, Tin-Tin Kyrano, Lady Penelope and Parker representing International Rescue as guests of honour on Skyship One's round-the-world maiden flight, unaware that master criminal The Hood is once again plotting to acquire the organisation's technological secrets. The Hood's agents murder the airship's crew and assume their identities to lure International Rescue into a trap. Meanwhile, Brains' efforts to design a proposed sixth Thunderbird collide with fate when Skyship One is damaged and its occupants' only salvation seems to be Alan's old Tiger Moth biplane.

References:
https://en.wikipedia.org/wiki/Thunderbird_6