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Six contestants have battled all the way to Finals Week, and as punishment for their hubris they are sent to Tasmania. Once upon a time this island was the destination for the worst of the British Empire's hardened criminals, but at least they didn't have to cook oysters when they got there.

On a freezing desolate shore, the judges greet the final six and inform them that they are there to re-enact Series 6 of Vikings. But first, they must walk out into the water and meet Giles, a local lunatic who has dedicated his life to the cultivation of aquatic snot. Giles teaches the cooks all about oysters, and they are fascinated to learn that oysters are actually extremely uninteresting. The contestants must then take six oysters out of cages where Giles keeps them as slaves, and cook them.

Julie has immunity today, so she receives the greatest gift any cook can get: not having to handle oysters. The other five must first present their six oysters, dressed in any way they like: smart casual, evening wear, swimsuit, whatever. They have half an hour to do this, and the worst three dishes will go into round two, where they have 75 minutes to cook more oysters however they want. The worst cook in round two will be sent home, which isn't that terrible a prospect given they're currently in Tasmania.

The challenge takes place at Devil's Corner Winery, named after the fact that Satan lives there. They have to cook out in the open air, which is terribly crisp and bracing and kind of stupid because why would you? The five hopefuls immediately begin doing whatever cooks do when they try to make oysters vaguely food-like.

Just a few seconds into the cook, anyone playing the MasterChef drinking game is forced to take a shot when Sarah mentions Indian flavours. Meanwhile Keyma is shucking an oyster for the first time ever, and discovering just how worth it the process isn't. As the only contestant willing to tell the truth – that oysters are revolting – Keyma is at a disadvantage, because today's challenge is centred heavily around lying to yourself and others. Everyone else, going along with the pretence that oysters are somehow good, is happily shucking their brains out.

Half an hour goes pretty quickly, although it feels a lot longer because we have to listen to Sarah talk about her growing confidence and classic French techniques, an experience which always results in massive time dilation. However, eventually the challenge is over, and the judges put their lives on the line by tasting the oysters.

Billie has dressed her oysters in chicken fat. They are disgusting and the judges love them. Daniel has dressed his oysters in smoky soy sauce and coconut vinaigrette. They are utterly foul and the judges love them. Alvin has dressed his oysters in brown butter. They are nauseatingly awful and the judges are like, "eh, whatever". Sarah has dressed her oysters in pretentiousness. They are completely horrible and the judges think they're kind of nice in a way. Keyma has dressed her oysters in…I dunno, something yellow? They are vile. The judges are confused. "Had the passionfruit been more tart and less bright, that might've given us a greener underpinning," says Melissa, proving she's been out in the cold too long and has become delirious.

In the end, Dan and Sarah are safe from round two, leaving Keyma, Billie and Alvin to celebrate oysters in a dish, a task akin to celebrating woodworm in a log cabin. Keyma begins by having a soft-focus flashback, wherein it is revealed that in Venezuela she was an engineer, but for the sake of her family gave up her job and made the supreme sacrifice: touching oysters.

Billie is disappointed: she was hoping to win in round one, but not for the first time her inability to be Sarah cost her dearly. She gets working on an oyster custard, which is as horrifying a combination of words as anyone has ever heard.

"This is a very interesting cook," says Andy, hoping to convince himself. Melissa asks the other judges what they would cook if they were in round two. Both Andy and Jock agree that they would do something with oysters. Melissa agrees that this would be smart.

Alvin is making an oyster omelette, which doesn't sound quite as nightmarish as oyster custard, but close. "It's so pedestrian, yet tasty," says Alvin, knowing that pedestrian food is always a crowd-pleaser. Andy ramps up the pressure by telling Alvin that his dish is Melissa's mum's "death row dish" – the dish she would cook if she had to execute a criminal using food. Can Alvin make his omelette genuinely lethal?

Keyma is stressed, as she's never made her dish with oysters before. "I have a heavy feeling in my stomach, and it's not a nice one," she says, suggesting that she's actually been eating her oysters instead of cooking them. Meanwhile the judges confer again. "You know what the great thing about this is?" says Jock. "It's oysters," he answers himself, summing things up fairly neatly.

Alvin has all his elements perfectly prepared and it feeling pretty good. "I'm in a good place," he says, having forgotten he's in Tasmania. Suddenly disaster: his wok is not heating properly. This is because they are cooking out in a field in the Antarctic circle, a location selected to test the contestants' ability to handle an environment in which no professional chef ever has to actually work. Possibly Alvin has chosen the wrong dish to make in a freezing field.

But at least he has the consolation that Keyma is also panicking. The taste of oysters is not coming through strongly, which is obviously a good thing, but nevertheless she is worried. She begins to panic. Melissa comes over and grabs her wrists, which some might consider comforting I suppose. "You are a woman," Melissa tells Keyma, and finding this impossible to argue with, Keyma gets on with it.

"You're on the home stretch!" yells Melissa with ten minutes to go: a phrase that lacks the puckish humour of previous shout outs like "You're coming out of your shells" and "Why don't you all get shucked?"

Meanwhile Alvin appears to have made some scrambled eggs. "It's all merging into one big claggy mess," he says. Luckily, this is an oyster challenge, and oysters are, inherently, big claggy messes. What better way to celebrate revolting gunk than by mixing it into an even bigger pile of revolting gunk?

Billie is very happy with her work. She is almost finished and her oysters smell like smoke, which is at least better than smelling like oysters. Also, unlike the other two, she has not undergone a full-scale panic attack in the last hour, so she's sitting pretty. She does admit that there's a risk the custard could be undercooked or overcooked, but since this is also true of literally everything that has ever been cooked anywhere, it's probably not worth worrying about.

The cook is over, and it is now time for the judges once again to pretend that oysters are food. They eat Billie's custard, which she worries is too firm. Jock thinks it's fantastic. Billie's oysters are a triumph, and everyone agrees that considering the inherent repulsiveness of the dish, it's great.

Next, Alvin. He breaks down in tears as he admits that he is always surprised when the judges like his food, because he has been trying to poison them since episode one. They taste his oyster omelette. It is not good: the too-cool wok has struck a terrible blow.

Finally, Keyma, hater of oysters and therefore the only sane person on the show. Her dish is an oyster pisca andina, which is Spanish for "I wish I didn't have to put oysters in this". She too breaks down in tears, as she thinks of her family and how much they also hate oysters, and how much she misses being in an environment where there are no oysters. Melissa tells her that it makes her extremely happy to see Keyma cry. "There's a lot that's gone into this dish," says Andy. There is then an ad break, following which, Andy says, "There's a lot that's gone into this dish." The judges eat Keyma's oysters and declare them wonderful beyond their wildest dreams.

Sadly, this means it's the end of the road for Alvin, but at least he has the consolation of knowing he was only eliminated after being forced to cook something that is not technically food. He also has the consolation of not having to stay in Tasmania, so overall it might be a win for him really.

Tune in tomorrow, when hopefully we can all leave the memory of adults voluntarily ingesting invertebrate mucus behind us.