Ah, the fifth episode. Traditionally where sitcoms stick their worst episode in the run - the idea being that after four (hopefully) excellent episodes you're allowed a bit of a duff one, but you want to leave the audience on a high in the final week. Adding to this was the episode synopsis: "Can the Lab Rats stay up for seven nights in a secret underground government facility?" So, the gang have left the laboratory for the first time... a break in format. Did they pull it off - or, after four weeks of pretty consistent episodes, have we got our first genuine dud of the run?

Nope. They've produced out-and-out the best episode of the series so far.

(A warning before we go on - this review contains major spoilers. Lab Rats hasn't generally warranted a spoiler warning before now - and hell, we're nearly two weeks on from broadcast now - but this episode definitely needs one, for reasons that I can't go into without them becoming spoilery themselves. So if you intend to watch this episode at any point - and I really suggest that you do, as it's a corker - then don't read any further. You'll regret it.)

The premise for this episode is simple - as mentioned above, the Lab Rats have to spend seven nights in a top secret government facility doing experiments for the university - as well as doing day shifts back at the lab. We see three of those seven nights here, and they roughly follow the same pattern of events. What we get, in effect, is three ten minute episodes, with tons of running gags - an original structure for sitcom, and it works wonderfully well. Lots of Rule of Three jokes here.

Before we get to the comedy, a word about the set. It's lovely. Beautifully retro-styled, stuffed full of blinkenlights and reel-to-reels, this isn't a piddling little extra set, stuffed in the corner of the studio - they've clearly taken over the whole studio for the thing. There's also lots of areas for the cast to do things, so you never get bored looking at the same set for an entire episode. I've heard a few snipes that it looks cheap and nasty, but I really don't think it does - it's stylised, certainly, but that's clearly very deliberate, and it completely suits the kind of show being made here. And the large pillars with neon lights at each corner are really striking, and make the set feel huge. A triumph for designer Dennis De Groot, and my favourite sitcom set in years.

Indeed, the set contributes to the lovely atmosphere of this episode. One set - no establishing shots of the location, or even an initial first scene back at the lab. Just five actors on one set, and nothing else. This is effectively theatre, and could be done as a play with minimal script changes - and I've always said that television these days should be taking more inspiration from the theatre. (The emphasis these days is on making television cinematic - which is fine in itself, and absolutely valid, but it means that other ways of making television are being forgotten about.)

So: lovely sets, check. Great atmosphere, check. The jokes? Indeed, another check - this is without doubt the funniest episode of Lab Rats so far. There's too many excellent jokes to mention - and my bumbling efforts at explaining them wreck them anyway - but my favourite has to be Alex explaining to Cara how the Spartacus Machine works:

BEENYMAN: RIght, look, it's easy. You put this left peg in, this right peg out, in, out, in, out, you take the next two out, you watch the chokey closely as you turn it round, and that way it all works out.

It loses something written down (and Addison performs it superbly, especially the last line), but it truly is a lovely joke - and one with a highly unusual structure. Lab Rats seems to have been dismissed as a show that does nothing new - and yet, for all the stuff that is slightly predictable (yet often still amusing), you get moments like this. Incidentally, the same joke is repeated later ("And this little peg goes 'beep beep beep beep' till it's turned on...") , and - as part of the aforementioned Rule of Three - you would expect there to be a third instance on the final night. This doesn't happen, however - perhaps it was edited out. There's certainly a moment where it could have slotted in.

This is also the best outing so far for Geoffrey McGivern as Professor Mycroft - both the performance, and the material. Up until this point, I've felt like the character has had huge potential but never quite worked - but not so here. Whether it's reminiscing about going to a lezzy fair, managing to make the same joke about his book funny every single time, or walking onto set dressed in a leopard skin posing pouch and absolutely bugger all else, he's an utter joy this week. Indeed, it feels like this is what his character should have been like all along - an emphasis on the 'dirty old man' stuff works.

MYCROFT: Textbook. Every time without fail, this is how it all starts. It's all in Chapter 2: "How it all starts". I've lost so many colleagues on all-nighters. A little friendly banter over a chair here, a joke about who slashed the contamination suit there, and before you know it, you're on a Witness Protection Programme in Kent with a wonky-nosed wife who looks nothing like the one in the brochure. Never pick the woman under the staple.

And then we get to that ending. That glorious, glorious ending. (Seriously - if you've not watched it, turn back now.) The Dean at the very start of the episode worries that the whole thing will end up as a "circus" - and the episode ends up with the five of them dressed up in circus gear. But how? Well, Alex is in his clown outfit which is actually an anti-static suit, as he's been getting static shocks all episode. Cara is on the trapeze, as a cumulation of her various ways to reach the controls for the Spartacus Machine. Mycroft is in a weightlifter pose - because his posing pouch and lamp are all part of making himself comfortable for the seven-nighter. Brian is holding a whip as a lion-tamer - or, actually, the lead for the electric fence he's using to protect his biscuits. And the Dean comes in dressed as the ringmaster - actually, another bit of horse riding gear she's been wearing throughout the episode. And that's not to mention the lovely little touches, which really sell the moment - Alex is rosy-cheeked with mad hair like a clown, because he's just been electrocuted by Brian's fence. Mycroft is actually in a weightlifter's pose, because his back has seized up. And the flashing lights from the experiment starting match the lights you might get at a circus performance!

And I can quite honestly say - I didn't predict it. I didn't have a fucking clue what they were up to. The whole episode was designed to lead up to this point - and I had no idea until the Dean walked into the room. And nobody I've talked to guessed the ending more than a few moments before the Dean walked in - the earliest I've heard somebody getting it when Dr. Beenyman dressed up in the anti-static-cum-clown-outfit, but most people I've seen - including me - only got it on the actual reveal of the Dean herself. Looking back on it, as soon as Cara got on the trapeze I should have got it... but I didn't. It seems nobody did!

The reason why nobody guessed is simple: it's set up immaculately. Both me and Andrew have complained about the structure of some of the series in our previous reviews, but there are no such issues here. For a start, the circus mention was set up properly near the beginning - enough so you'd remember it, but not beating you round the head with it. It's then mentioned once more, and then... nothing. You're not constantly reminded about it, like less deft - or indeed, more cowardly - hands would have done. Set up so you'd remember it, but then trusted not to forget - treating the audience with intelligence.

Secondly, the individual plot strands work well by themselves. Dr. Beenyman and the static, Cara and the Spartacus Machine, Brian and the biscuits, Professor Mycroft making himself at home, and the Dean with her horse outfits - these elements all have their own jokes, and they feel completely unrelated. It doesn't feel like setup - it feels like the actual material. Of course, it ends up being both, and that's the brilliance of the episode. (Note, too, that the wide shot which punctuates the climax is set up with all the wide shots at the start of each section of the episode - which, again, really helps sell the gag. A beautiful bit of directing. Indeed, the directing and editing seem to have taken a step up this week - there's an interesting top-down view of the set at one point, and a great cut-to-black moment with Cara taking a running leap at the machine at the end of the first section.)

Finally - the whole emphasis of the show seems to be 'staying up for seven nights'. So you think that the climax will somehow relate to that - on a very basic level, say, somebody's tiredness causing requisite chaos. So the actual ending is something that's completely out of the blue - nobody's mind is thinking about the circus stuff at all, but wondering how the tiredness stuff is going to pan out. It's deliberate misdirection, from the title of the episode onwards. Superb.

Criticisms of the episode, then? Only a couple of minor ones this week, to be honest. Brian fancying the Dean still isn't working for me, which is a shame as it feels like it's supposed to be a major point with the character. There are also still a few problems in the technical department too - the odd misframed shot and the like. I have to say that it doesn't really bug me, except in one instance - Mycroft's picnic basket. The whole point of the gag is that with one flick of the wrist it opens up to reveal an entire picnic, tablecloth and all - but it cuts back too late, and misses the actual opening of the basket itself! Whether this was deliberate decision, or just a bad bit of vision-mixing that wasn't fixed in the edit I don't know, but it buggers up the gag a tad.

Still - my misgivings can be fitted into one paragraph this week, and neither are especially important. Lab Rats has tootled around for much of this run as nice, fun sitcom, without reaching the heights that it should have. Then, with A Protest, the ending reached the heights that the series was clearly capable of. This goes beyond that - this is a stunning climax, of the kind that I just didn't think the show could do. It's one of the best endings to a sitcom episode that I can remember - and I watch an awful lot of sitcoms. This is a series that has strained to be Father Ted in tone, and struggled slightly - and whilst the episode as a whole isn't of Ted standards yet, the ending manages to surpass the ending to any Ted episode. No mean feat.

Throughout the whole of this series, I've wanted this show to be recommissioned - it's exacty the kind of show I like in style and intent, I was getting laughs from the show, and the potential was always there for something greater. What I wasn't expecting was for this potential to be realised this series - and this episode manages it. The show deserves a second series now - and if it doesn't, the BBC will be chucking away one of the most promising sitcoms they've made in years. A bloody good half-hour of sitcom - and whilst the climax is the real story here, let's not forget that the rest of the episode took a great leap forward too.

The only question is - why the hell was it hidden away in the fifth episode slot?