The murmuring is upbeat and expectant. It is like walking into a classroom to discover that your maths teacher is off sick, and the replacement they've brought in specialises in musical comedy. It shouldn't feel like that, of course, but judging by the buoyant noises that greet this afternoon's stand-ins, it does.
The Labour deputy leader (the correct and appropriate position for this task), Harriet Harman, who wasn't really having a good week already, was, it was announced at the last minute, facing recognised middleweight debating champion, the shadow foreign secretary, William "Bruiser" Hague.
If she has come dressed for the battle we cannot quite tell. The padding underneath her beige jacket is barely discernable and, if anything, quite shapely. This womanliness is noticed by the ever-sharp Hague, who points out that the right honourable member should be congratulated on being female, especially as the first woman to stand in that position in 30 years ... since his dearly beloved Iron Maid.
An affectionate-but-pointed poke is enough, apparently, to kick off the fun.
Soon the important matters of foreign affairs pertaining to be the present question at hand - you know, it's only Zimbabwe - give way to inter-party carousing and general horseplay either side of the box.
"I would ask him why is he asking questions today, because he is not the shadow leader of the house. The shadow of the leader of the house is sitting next to him," Harman says, in response to the shadow foreign secretary pointing out his awareness of her femininity.
And with good point, of course. The shadow leader of the house is Theresa May, sitting down, quietly but with massive grin, behind Hague. Still, according to Debrett's, the polite thing to do if someone points out your gender in a public arena is to thank them politely and go and find a policeman, but the level of debate, we are to understand, is beyond etiquette, and far into the realms of sexual politics.
"Is this the state of the modern Conservative party, that women should be seen and not heard?" Harman asks, pointedly. "If so, I offer some sisterly advice to the shadow leader - that she should not let him get away with it!"
If there is a sisterly tear of gratitude rolling down May's cheek, we cannot see it from here. The camera cuts away too quickly to Hague, round head now held back and poised to bowl full speed toward Harriet's already wobbly pins.
"Turning to domestic issues," he rumbles, lightly, as the roll begins, "I was going to be nice to the leader of the house. But now ... speaking of dressing for the occasion, if she goes to a building site she apparently wears a hat, if on police patrol, a flak jacket - presumably if the right honourable member goes to a cabinet meeting she dresses as a clown!"
He looks around as colleagues mournfully mumble their praise with the traditional creaking noises. To be fair, it wouldn't win Hague a freestyle rap battle, but it does the job.
"If I'm looking for advice on what to wear or what not to wear, the very last man I would turn to is the man in the baseball cap," Harman responds.
It would surely be clichéd to suggest that there is an element of flirtation simply because one of them happens to be female, but the argumentative bounce of Hague does particularly seem tinged with an edge of Spencer Tracy to Harman's Katharine Hepburn - a similarity which is, admittedly, noticeable in part due to the uncommonly spherical heads of the two men, rather than the level of dialogue so far evidenced.
"Turning to your actual important question ..." Oh why bother? Spare us the pretence, we're here for the sparring – but for another minute they actually keep to the business at hand.
Hague, proving himself to be as internet-savvy as we always suspected he would be, quotes a blog post from two months ago in which Harriet claimed that people were not as economically worried as they are now. Well, if we're going to be internetty about this, then surely he could have just left a comment?
"Rather than worrying about my blogs ... He should be worrying about his income as an after-dinner speaker!" returns Harman, with an aimless arrow that seems mainly like the kind of rather empty insult one might perhaps scribble on the corner of one's notes in the corridor beforehand as an emergency measure.
More politics interrupts the verbal spar. Before being returned, pointlessly, to the same old opposition question.
"And isn't the question the whole country's asking: when can we get rid of this discredited cabinet and have a change of government?" Hague ends.
The jeers and yeers rise in volume, and drop away again as he sits. "Yer never got there, did ya?" shouts one uncouth heckler, clearly bored with farmyard disagreement noises.
The questions continue, the banter diminishes to a barely discernable level of snark. Questions are asked, and answered in beige-jacketed deputy leader fashion.
Vince Cable, met by rounds of hilarity and applause like a favourite end-of-the-pier magician appearing for the encore that everyone was waiting for, nods and smiles and holds his fingers up, in a "No, just wait for this one" kind of way. And then asks a question about the Queen postponing her anniversary celebrations.
"Does the right honourable member share my view that her majesty is showing an unerring understanding of national mood, or does this government think that she is overreacting?" with a nod and almost a wink.
At this, Michael Martin's jovial smile, the one that has been clearly nodding along with the fun, is strained at the edges. He is, as he addresses Cable, a slightly more puce shade of berry than usual.
"The honourable member must not discuss her majesty the Queen in the house. Does he want another question? Because he's not getting a reply."
And after wasting his enormous build-up on a clever but powerless punch, Cable, clearly deflated, backs off and asks something dull and important about money.
After the first 15 minutes the heat is already gone from the fiery first flush of the great Harman/Hague screwball question time.
The main news stations even give up, swapping PMQs for real actual breaking news about the Zimbabwean elections. When we come back on BBC Parliament, everyone is looking slightly deflated and annoyed that the fun didn't last and they had to hand in their homework after all.
"Is the government aware that when they changed the emphasis from the retail price index to the consumer price index, that their new index would be lower than the high one, and something something something pensioners?" Asks one Conservative minister, hoping to corner Harman and win the day.
"Um. Er. I will ask ... the chancellor ... to write to you. Answering that," says Harman, before, just in time, the bell rings, and everyone tries to get to the canteen before the chips run out.