The jungle of Guyana. They're spurred on by Mark Billingham, a jungle warfare instructor who served with special forces. They also face challenges inspired by a tribe of former cannibals.

Norwegian School of Winter Warfare in Norway. They are placed in the vicinity of a TNT explosion and forced to jump into an ice hole while still wearing their skis. They were required to dig a snow hole and sleep in it (something Norwegian school children routinely do at winter camp, though they get to use a sleeping bag).

The Royal Navy. After a week's training in Portsmouth, they go aboard HMS Belfast on nearby Whale Island and face simulations of both a sinking under heavy gunfire and a terrifying helicopter crash. After disorientation and freezing temperatures, their final challenge is a two assault course challenge including the disassembling of a 1,250lb gun.

Sahara Desert. They find themselves at the mercy of a former Foreign Legion instructor. Taking them into the Sahara, he subjects them to a series of gruelling challenges, including being buried alive, a 10-kilometre camel race, and an exhausting run over the sand dunes while wearing stifling gas masks.

US Navy SEALs. They travel to Virginia to carry out "simulated" Navy SEAL training with some former US Navy SEALs, whose beach-based challenges from long beach runs to "surf torture" leave them physically and emotionally drained. After the first day Carla suffers from dehydration and exhaustion, but must face her fear of water in a drownproofing challenge where the volunteers repeatedly sink in a pool with their ankles and wrists bound.

Everglades. They will be sharing the territory with two million alligators and 60 species of venomous snake, and perhaps most formidable of all, their Native American guide. “My name is not important to you,” he sneers. “But you will call me Teacher.” They have to cut open an alligator and remove its entrails with their bare hands. They canoe across alligator infested waters and wrestle alligators, and play a game of lacrosse.

South Africa. They are required to take on the might of highly trained Zulu warriors in a series of stick-fighting contests. However, before they are deemed worthy of battle, the contenders must first prove themselves by stealing honey from wild bees, and participate in a body cleansing ritual.

The unbreakables are handed over to The South African Elite Fighting Force, ex-special forces instructors who teach African armies the reality of warfare. The unbreakables are woken before dawn to find that Barnaby is nowhere to be found.