Backstage, Giles, carrying two weights, tells Marc, the magic act kid, "if you, you calibrated the units of weight then you could calculate the, uh, specific maximum velocity achieved before the point of impact." Magic Boy tells Giles that he's "smart" and asks him to do him a favor." Da Da Dum.
Back at the library, Xander points out the obvious by asking Willow what the square root of 841 is. After answering correctly, she gets the point, but Buffy reassures her that they will keep her safe.
Backstage, Marc is demonstrating his guillotine on a honeydew melon and then tells Giles to lie down and he'll show him what he needs to do. Marc's assistant has mysteriously disappeared, and Giles has reluctantly agreed to stand in for her. Giles wants to know how the contraption works, but Magic Boy puts him off with, "A good magician never tells his secrets." As he's fiddling with the rope, we see that his hand has started to change to the demon form. I should have known Marc was evil as soon as I saw that ugly patchwork vest. Only a demon would be clueless enough to attend high school in a horror-show garment like that.
Back in the library, Buffy and Co. are still trying to puzzle out the identity of the demon. They finally remember that Giles is also in possession of a juicy chess-club brain, and they hightail it to the auditorium.
Backstage, Giles is all set to get his head cut open. Marc padlocks the restraint around Giles's head. Giles is starting to get nervous now and asks Marc what the trick is. "'Trick'?" repeats Marc quizzically. Marc grabs an axe and proceeds to swing at the rope restraining the guillotine. This must be the dullest axe blade ever, because even after the fourth swing the rope is still holding, albeit by only a thread. Marc has the axe raised high over his head for the final whack when Buffy tackles him from the side. They fight, and as the demon begins to manifest itself in portions of Marc's face, Buffy says, "Ew," and lets down her guard a little. Marc takes advantage of this to get a few shots in. The rope begins to stretch and fray and finally snaps. At the last possible second Xander grabs the end of the rope, thereby stopping the blade. I bet Giles wishes that he owed his life to anyone but Xander. Willow is on the scene now and quickly hurries to undo Giles's restraints. She wants to know where the keys are, but Marc has them. Xander kicks her a hatchet and she begins to hack at the lock. Meanwhile, Buffy and Marc are fighting some more. She gives him a vicious kick and he stumbles into the box that was to be part of his magic act. Buffy wonders aloud how to lock it, but Marc bursts through the wood in full demon form. We of course don't see the actual transformation because they already blew the special-effects budget on the fake brain. Demon Marc grabs Buffy by the throat and slams her against the wall. Willow finally breaks the lock, and Giles sits up in order to remove the restraints from his feet. Sid shows up from wherever he was hiding (probably trying to make it with one of the cheerleaders behind the bleachers) and jumps onto Demon Marc's back, stabbing. Demon Marc loses his grip on Buffy, and she punches him a few times before kicking him and causing him to trip so that he falls neatly into proper head-chopping position on the guillotine. Giles shouts at Xander to let the rope go and thwack: no more head. Willow cringes and everyone looks around and takes a deep breath. Sid has made his way over to Demon Marc's body and is poised with his knife at the ready above the demon's heart. Buffy asks what he's doing, and Sid explains that if they don't stab him through the heart the demon will rise again. Buffy offers to do it for him but Sid says he can handle it. He stabs Demon Marc and then raises his arms to do it again, but slumps over the demon's body as the curse is lifted. Buffy picks him up and cradles him in her arms. At that moment the curtain goes up, and in one of the most priceless BTVS moments, everyone looks out into the audience surprised. Principal Snyder, sitting in the front row, says, "I don't get it. What is it? Avant-garde?" And the screen goes to black.