Tonight's episode begins in the MasterChef house, where Christina has donned her Dick Tracy's-girlfriend outfit, and Rishi informs us that we're in finals week. “How exciting's that?” he asks, fortunately never finding out what the audience's answer would be.

Today we are to have a mystery box challenge, the winner of which will gain “an enormous advantage”, according to Gary; bearing in mind that an enormous advantage, in MasterChef terms, usually means you're about to get eliminated.

In the mystery boxes is a bunch of food, which is pretty novel. George announces that at the start of the competition the contestants were asked what they would be if they were an ingredient. So it's going to be one of those really annoying days. The ingredients they chose are in their mystery boxes. Rishi chose chilli, for example, because he says he is “red-hot”, and chilli is a famously deluded ingredient. Emma chose coffee beans because they are inarticulate, and so forth. The rest of the ingredients are based on what the judges think of the contestants. They think Christina is like onions, because when you peel away her layers she makes everyone cry. They think Lynton is like kangaroo because he keeps children in a pouch. They think Rishi is like prawns because prawns tend to act very superior to other people. Et cetera.

They begin cooking. The judges discuss the contestants. Matt opines that it is all about momentum, a blatantly inaccurate statement.

George asks Lynton what he's making. Lynton tells him, producing a crash of percussion on the soundtrack, indicating that Lynton is making a terrible mistake. Gary and George ruthlessly undermine Lynton's confidence because they are jealous of how handsome he is.

Christina takes the opportunity to explain the rules of the challenge to us. For god's sake stop patronising us, Christina. She is making a ganache which is a chocolate thing or something I guess. Meanwhile Emma is making kangaroo with about twenty different things made out of kale. This could cost her if the judges find out that “kale” isn't a real thing.

At Rishi's bench, George and Gary arrive to crush his dreams. They investigate his mayonnaise, if you know what I mean. Something is not right. “This is a disaster,” whines Rishi like a big fat drama queen. It's just mayonnaise, you know?

Lynton wants to push himself to prove that he's worthy of being in finals week. Yet doesn't he know he is already worthy of being in our hearts? He'll do better than Samira anyway, Little Miss Runny Puree, who has lived up to the nickname I just gave her by making her puree too runny.

George yells something but I've tuned him out by now so I don't know what it was.

Matt goes to Christina's bench to tell another lie about momentum and point out to her that she must be insane to think she can make her chocolate tart thingy in the time available. He adopts his best “oh you poor idiot, you're dead and you don't even know it” face, but at least Christina has a plan: Rishi doesn't even know what he's doing as he throws out his mayonnaise and flings his prawns about and just generally has an aneurysm.

With four minutes to go Emma has only cooked one piece of kangaroo and has a blender full of what appears to be the waste product of an extremely sick baby. Meanwhile Samira has grabbed her prawns, so to speak. The tip of one of them is undercooked, but Samira doesn't mind: she considers it OK to come the raw prawn if it's just the tip.

Time is up. “This is pretty much what I pictured in my head at the start,” says Lynton, looking down at a plate filled with a sad indictment of his limited imagination.

Judging time, and Gary tells the contestants how much fun it is to cook with ingredients that represent the contestants, tastefully glossing over the fact that it is actually really stupid. Samira is up first, serving a bowl of prawns floating in snot, and the judges immediately gasp in horror, as the faces of those they've wronged float in the soup. Also there's a raw prawn in there, which Samira knew about but thought it was OK because she doesn't understand how food works.

Rishi is next, with delicious chilli prawns. He really IS red-hot! But so boring we move on within seconds. Lynton steps up and is also pretty dull, but when Christina arrives, you can tell just by the string section that something dramatic is about to happen. Her dish is tasty, but also very simple – but then again, they were supposed to be representing themselves so I think she fulfilled the brief.

Emma, on the other hand, has presented a “cracking dish”, according to Matt. He likes how she has presented “all the different sides of kale”, and I can only assume he's cruelly mocking Emma, because kale has no different sides and really it shouldn't be food at all.

Gary notes that the challenge divided the contestants into two groups: the ones who did really well and those who didn't. His insight is astounding. Anyway Emma wins which was pretty obvious, and because she won she gets to choose the protein for the next challenge, which will determine who will get an advantage for the elimination: which is, as expected, not “an enormous advantage” at all, but a very minor advantage, that five minutes into the next challenge will clearly be not one at all.

George reveals the options: barramundi, which provides a chance for Samira to reminisce about how she was really stupid once; beef, duck and squid.

“Emma, you have a huge advantage,” Matt lies. “There's a lot riding on this,” Emma also lies. Matt explains how minor and unimportant this advantage is, and Emma picks barramundi, because she has lots of ideas about fish, which is a bit weird.

Off they rush into the kitchen, to stare at delicious Western Star butter, available at your local Coles. Emma gazes at the butter for ages, trying to see into the butter's heart, and ponders her foolishness in having ideas about fish – none of the things she wanted are there. Maybe she could just stuff her barramundi with delicious Western Star butter?

“Let's hope you're not the fish out of water!” cries Gary, who is quite simply the worst kind of person.

Christina, having learnt her lesson, has chosen not to make her barramundi into a chocolate tart. A wise decision, or has she flown too close to the sun this time? Judging by the colour of her dress, yes.

George tells Samira her barramundi must be crispy-skinned. Samira nods and smiles. George doesn't like her smiling. “Focus!” he snarls, furious at the appearance of happiness in his presence. Samira explains in detail her activities regarding purees and radishes. “I sound so fancy, don't I?” she giggles, apparently having smoked some powerful substances earlier.

The judges congregate to pretend that barramundi is interesting in some way. Meanwhile Emma has found that her beurre blanc isn't really beurre blanc: she has failed to do justice to the delicious Western Star butter, which is both high-quality and affordable. Gary tells her she needs more delicious Western Star butter.

Lynton is worried about his fish: it seems listless and depressed. He tests his fish with his knife: it's touch and go. Lynton calls a vet.

“I can't believe it, it's just around the corner!” George yelps in the mistaken belief anyone wants to hear his voice ever again. With a few minutes to go Samira is putting her skewer into the fish and touching it to her lip, a special technique George taught her for when she wants to look a bit of a prat.

Over at Emma's bench everything is going wrong, but Emma is still smiling as she says something or other that I didn't catch. Lynton is far less happy, as his barramundi remains downcast, but that's quickly forgotten as Emma attempts to self-immolate.

The fire doused, time is up, and Emma suddenly realised she's forgotten it's an invention test. Her dish isn't inventive. Conversely, Samira's IS inventive, but looks weird and horrible. Swings and roundabouts.

Rishi knows his dish isn't inventive enough either: he serves his crispy-skinned barramundi tagine and the judges think it's wonderful. But Matt notes its lack of originality and makes clear to Rishi that the cruel and arbitrary nature of fate could see him crash.

Up steps Emma with her barramundi in a yellow puddle, for which she apologises. Fortunately the fish and the puddle are fine, but again, she has been boring and safe, this time with her cooking.

Up steps Samira, whose to-camera cutaways are becoming more and more drunkenly manic. She hopes her fish is cooked, as does the conductor of the MasterChef orchestra. We know she's done OK though, as the portentous strings give way to bubbly plinking and her dish turns out beautifully.

Lynton's turn. Lynton wants to win MasterChef 2013, but cannily identifies the crucial issue as to whether he can cook well or not. He smiles at the judges, which for most people would be enough to have him awarded the prize then and there, but these are hard, loveless men, and he will be judged not on his wonderfulness, but on his barramundi. Which is overcooked. NO. NO. NOT LYNTON.

Lynton is kicking himself, and the absurd, surrealist nature of the show is brought into sharp relief as we see a grown man devastated beyond reason by an overcooked fish. Is this what we have become, society?

Last is Christina, who has cooked, in George's words, “bacon, barramundi, mushroom, BANG!” indicating his desire to shoot her. “Boom boom, you've shook the room,” says Matt, sadly misjudging the public's thirst for downbeat George Calombaris impersonations.

The best dishes were Christina and Samira, who are called forward fairly pointlessly, as it is immediately announced that Samira was the best, thus explaining why in all the cutaways in this episode she had been acting like a puppy on cocaine.

And so Samira gains what is sure to be an extremely small advantage in tomorrow's elimination, which will be conducted under the watchful and expert eye of some bald dude. Can't wait!