Thunderbirds had made a major impact in British TV, leading to the 1966 theatrical release Thunderbirds Are Go, but disappointing box office returns and lack of interest from major US networks (the show was syndicated in the US instead of picked up by one of the three big broadcasters) caused cancellation of the series. Despite this, one more theatrical film was prepared, and the resulting film has proven to be a highlight of the Thunderbirds epic despite once again suffering from disappointing box office returns.
Thunderbird Six is a vast improvement over Thunderbirds Are Go thanks to a more coherent plot and more plausible action scenes; also adding to the film's quality was the decision to tone down the action scenes in favor of more character interplay.
Most of the cast returned for this final go at the Thunderbirds epic; notably missing aside from David Holliday (replaced by Jeremy Wilken after the first season of the show wrapped up) was Ray Barrett; added to the voice cast was Geoffrey Keen, later to win fame as HM's Defence Minister Sir Frederick Grey in the James Bond series.
"Brains" Hackenbacker (David Graham) has been brought to New World Aircraft, his identity hidden, to make a proposal for a new machine of flight to the company's board of directors. Brains' proposal is to go back to the future - to the era of the passenger dirigible.
Brains' idea is laughed out of the company, but they turn around and build it anyway - Sky Ship One. NWA has invited members of International Rescue for an exclusive round-the-world maiden voyage before the ship enters full commerical service, but Brains is put in charge of creating a Thunderbird Six for Jeff Tracy - an assignment that begins to stress his relationship with the Tracy family.
Much of the film makes use of a reallife Tiger biplane in flight, the Tiger being a special retro project created by Alan and TinTin. With very heavy security, the four IR members invited to the flight of Sky Ship One - Lady Penelope, Parker, Alan, and TinTin - are escorted to NWA's flight base and board the ship.
But unknown to the members of IR as well as to NWA, the crew of the fully-automated jet-powered dirigible have been assassinated and replaced by a band of killers led by a man named Foster, working for a kingpin known as Black Phantom - in reality The Hood wearing a bad hairpiece.
The film then follows the voyage of Sky Ship One as the killers work to position a perfect trap for their passengers as well as International Rescue itself. Along the way Penny, Parker, Alan, and TinTin overfly and visit numerous locations visited by IR in previous rescues - the Atlantic Ocean, New York City, the Grand Canyon, Africa, the pyramids of Egypt, and the Swiss Alps. Here we for once get to see the members of IR able to relax and enjoy each other's company, unaware of any danger to their safety - or are they?
The visits to varied international locales add nicely to the film's atmosphere and allow the buildup not only of suspence but also allow appreciation of the interplay between the characters; this makes the inevitable action scenes and rescue mission all the more gripping and suspenseful as disaster strikes and Scott and Virgil Tracy launch into action.
Without question this is a zenith in the International rescue epic and ends its initial run on a high note.
