Welcome to our Thunderbirds Deep Dives! As we celebrate International Rescue's 60th anniversary, we asked you to pick your favourite episodes of Gerry and Sylvia Anderson's classic 1965 sci-fi adventure series that should receive in-depth, analytical retrospectives. Based on your picks, we've collated a top 10 selection of Thunderbirds greatest episodes to receive a review - as voted for by you!

Our Thunderbirds Deep Dives near their end with Pit of Peril - the first ever Thunderbirds episode which had to be rapidly reproduced from 25 minutes in length to 50 minutes. In this classically regarded adventure, the colossal army dredging mech Sidewinder inexplicably collapses into an underground inferno. Can International Rescue salvage the vehicle before the crew perishes?

Debut transmission: 7th October, 1965

Writer: Alan Fennell

Directors: Desmond Saunders

Perilous plotting

Pit of Peril, Thunderbirds' second episode to be produced (and second episode to have been voted for by our fans in this top ten countdown, by sheer coincidence!), ought to pale in comparison to its predecessor. Pit of Peril borrows much of the more celebrated Trapped in the Sky's narrative rhythms with seemingly little creative excursions to enable the episode to stand on its own merits. Both episodes detail the magnificent debut of some status quo-shifting super-mech, which swiftly falls into disaster. A rescue is attempted but ultimately fails, necessitating the intervention of International Rescue, in particular Virgil Tracy's nimble piloting of uniquely-built pod vehicles to save the day. As we'll explore, Pit of Peril's merciless retreading of that previous episode may have to do with its tricky production history.

Pit of Peril is a far more mechanically and visually aggressive adventure (heightened by its stark jungle setting) than Trapped in the Sky, but where Trapped in the Sky eloquently lays out the core format of Thunderbirds, Pit of Peril follows much the same structure - and fails to impress as much. On close inspection, however, some sparkling elements of the episode gift it the ability to stand further apart from Trapped in the Sky as its own serviceable adventure.

Fennell's debut

Pit of Peril was Thunderbirds' first episode written by Alan Fennell, who liberally copies the format of Trapped in the Sky. Future episodes from him - Brink of Disaster, Terror in New York City, Martian Invasion - all have greater fun unravelling Thunderbirds' stereotypical narrative structures, imbuing them with distinctive personality within Thunderbirds' wider canon. Here however, he plays things quite straight-faced, as if he's adjusting himself to the demands of writing for the series.

Compared to Trapped in the Sky's gradual detailing of the Hood's sabotaging of the Fireflash, mere minutes into Pit of Peril delivers the spectacular disaster that befalls the menacing Sidewinder. Throughout Trapped in the Sky, the combined threat of the Fireflash's crash-landing and rupturing of its atomic reactor delivers much of the tension, whereas in Pit of Peril, we cut out such build-up entirely - an indicator of the episode's 25-minute genesis, perhaps? The Sidewinder's test trials through treacherous jungles end in disaster when the vehicles collapses into a barely concealed dumping ground of WWII military cast-offs, spontaneous infernos engulfing the stricken craft, its three-man crew trapped inside.  

Pit of Peril eschews any tense build up that characterises many other episodes of Thunderbirds and goes straight for that pyrotechnic money shot. The Sidewinder's collapse delivers our first tangible impact of Thunderbirds' techno-advanced future going haywire. By design or accident, this is also a rare instance of International Rescue coming to the United States military's aid, while the Sidewinder's very purpose of clearing inaccessible terrain for military powers delivers a rare acknowledgement of war still being active in Thunderbirds' otherwise idealised futuristic timeline. The episode's opening scenes, interspersed with real-life documentary footage of wild animals, framed as if the animals are fleeing the Sidewinder's path of destruction, are harder to justify in our current age of climate change.

From a production perspective, it's absorbingly impressive to witness the military men, coordinated by General Peters aboard the Copter Watchdog, traverse the perilous pit when attempting to salvage the Sidewinder using their own poorly executed plans and equipment. The puppets manoeuvring through billowing smoke, thick rubble and scorching flames demonstrate how advanced AP Films' pyrotechnics and puppetry could be when blended together by the time Thunderbirds arrived. From a narrative perspective, however, they drag the episode down. There's nothing going on in these scenes that Trapped in the Sky didn't already attempt, and it would prove rare for future episodes to display this justification for International Rescue's existence. 

Brains' leadership

International Rescue's efforts in salvaging the Sidewinder delivers one of Thunderbirds most satisfyingly uncomplicated rescue operations. The carefully coordinated efforts of Scott, Virgil, and Brains makes for a modestly compelling effort that capture Thunderbirds at its least surprising. Awkwardly, Scott's arrival at the danger zone results in him mostly surplus to requirements until the heavy duty gear can arrive. Only two episodes in to the series, and Scott's mobile control unit is already difficult to justify!

Instead of guiding the rescue, Scott becomes as involved in the operations and Virgil and Brains. International Rescue's chief engineer in particular is framed as the authoritative intelligence in guiding Scott and Virgil's efforts in detonating the pit further to allow a clear passage for the Recovery Vehicles to drag the Sidewinder out. Later series 1 episodes such as Day of Disaster and Desperate Intruder would expand Brains' involvement in rescue operations to novel effect, while series 2 episodes Alias Mr Hackenbacker and Lord Parker's 'Oliday would evolve Brains still further. This early on in the series, we have a clear syphoning of roles - the Tracy boys get their hands dirty operating the vehicles, while Brains remains the calmy collected voice of reason.

Pit of Peril introduces us to the Mole, one of International Rescue's premier pod vehicles. Its inclusion in the series' end titles montage of Thunderbirds' most recognisable vehicles is a neat encapsulation of its importance. In rare exceptions, pod vehicles tended to make solitary appearances in Thunderbirds and were generally the invention of individual scriptwriters, rather than Gerry or Sylvia. The Mole, surely Fennell's invention, perhaps has its origins in his comic strip work for Supercar in the pages of TV Comic, which also introduced a burrowing machine of the same name...

Pit of Peril's meticulous editorial stitching together of puppet stages and special effects sequences enliven the drawn-out pacing of Scott, Virgil, and Brains' rescue operation. The swiftly edited back-and-forth between Virgil in the pit, Scott at the controls of the Mole, Brains stationed in the pod laboratory aboard Thunderbird 2, and Virgil's later place driving the Recovery Vehicles are all superbly paced that give visual energy and dramatic gravitas to the seriousness of the Sidewinder's rescue. Once again, it certainly feels as though Fennell is cribbing heavily from Gerry and Sylvia's efforts in mapping out Trapped in the Sky, which boasts a similarly edited climactic sequence when the Fireflash makes its emergency landing atop the Elevator Cars. However, where that episode prioritises the glamour of civilian airliners, Pit of Peril emphasises a darker, murkier world of jungle warfare and environmental jeopardy.

Indeed, the military's portrayal in Pit of Peril is one of many varying portrayals of the outfit throughout the series. Evidently, some substantial time has passed between the events of Trapped in the Sky and Pit of Peril. Where International Rescue unveil themselves to the world with their rescue of the Fireflash, Pit of Peril demonstrates that I.R. is a widely known entity by this point. International Rescue isn't treated with any disdain, suspicion or hostility by the military here as they are in later stories, such as the explosive events in Terror in New York City, The Imposters, or The Cham-Cham.

Fennell's liberal copying of Trapped in the Sky is perhaps most keenly felt in the episode's climactic moments - Virgil piloting one of International Rescue's rarely seen pod vehicles, complete with remote controlled drones, to rescue some super vehicle - sound familiar? With the triumphant dragging of the Sidewinder out of the inferno belly before it can rupture any further and subject the crew to fatal exposure, Pit of Peril neatly wraps up its blazing drama, even if such a visual spectacle struggles to push further than the electrifying airliner runaway finish of Trapped in the Sky.

A tangled production history?

Pit of Peril boasts some eye-opening lasting impacts within Thunderbirds. For starters, TV Century 21's readers viewers tuning into the episode for the first time in 1965 would have recognised the Sidewinder from the Stingray comic strip storyline Curse of the Crustavons, published between March and April of 1965. Curiously, the vehicle operates under a totally different name - the Jungle Cat. Why the name change? Was 'Jungle Cat' Alan Fennell's original name for the vehicle, sharing the animal-themed name conventions as the Mole? 

With the recent in-depth research from Century 21 Films that Trapped in the Sky was almost certainly produced as an hour-long episode, Pit of Peril then was the first instance of AP Films having to expand an established Thunderbirds episode from 25 minutes in length to 50 minutes. Research into this episode specifically by AP Films uncovers an abundance of fascinating details as to how the episode underwent significant changes from 25-minute produced episode to its transmitted version. The episode was heavily edited at the eleventh hour to 'correct' International Rescue's prolonged absence from much of the first half of an earlier version of Pit of Peril's hour-long rendition.  

Pit of Peril's association with Thunderbirds' comic book worlds would expand into the 1990s with its two-issue adaptation by Fennell and Steve Kyte in the pages of Thunderbirds the Comic. At just two instalments of four pages each, this comic version perhaps offers a close insight as to how the earlier 25-minute TV episode would have been paced.

The awkwardly cumbersome design of the Sidewinder and the smoke-induced state of the puppet stages made Pit of Peril an unpopular episode with much of the series' crew. Yet here we are at number #2 in our fan-chosen countdown of Thunderbirds' greatest episodes. The Sidewinder's memorable design balances between being mechanical ferocious and comically unrealistic. With its exotic setting, relentless underground carnage, and hardware-heavy spectacle, Pit of Peril may struggle to stand beyond Trapped in the Sky's shadow, but it remains a brawnily energetic early adventure for Thunderbirds.

Sign up to the Anderson Entertainment newsletter to receive all the latest Gerry Anderson news, exclusive releases and more transmitted direct to your inbox!