Walt argues his position on both ontological grounds (the fly shouldn't be there, thus "contamination" is accurate word use) and on the grounds that the purity of their meth is at stake. Jesse looks at the swatter and asks if it's Walt's "fly sabre." Heh. Nice. He also spots the band-aid on Walt's dome and asks if he hit his head very hard? Or perhaps sampled their product? Something to explain how crazypants he's acting right now? Walt starts to explain that in a "highly controlled environment" such as theirs, any contamination has the potential to... something-something. He trails off because the fly is in his sights. Jesse's like, "Seriously, but did you sleep? AT ALL? Because..."
A timer goes off on one of the vats of... meth juice or whatever. Walt says they're running out of time so "let's get started; the sooner we do it, the sooner we're done." Jesse's like, "Awesome!" and goes to get a bag of whatever to dump into the vat of stuff. (Oh, I know how to make meth. I am just being a responsible citizen by not telling you how it's done.) Anyway, Walt's like, "What do you think you're doing? THERE'S A FLY." Jesse looks at Walt like he's got 3.5 heads and tries to appeal to his meticulous nature, saying the timer went off, so it's time to make the donuts. Walt says the batch will be worthless unless they take care of "the contaminant." By the way, I hope you all are making the appropriate connections about contaminants and spoiling the whole batch with all the Jane remembrances we've been having this week. Come on, I can't do all the work for you. So they go back and forth on the subject for a while, until Jesse gives in and agrees to hunt for the damned fly already.
Or at least Jesse agrees to mill around while Walt hunts the fly. He does make a very good point in saying that they're cooking METH, which at its best is still a poison manufactured for burnouts who care about quality perhaps less than anybody else on the planet. But if you've been watching the show, you know how fiercely Walt holds on to the "pride of product" thing as one of his primary rationalizations, so that argument isn't going to hold much weight. So while Walt hunts, Jesse babbles about rat turds in hot dogs or whatever, while trying to surreptitiously mix some whatevers into the meth-y brew. I love the idea that Jesse is trying to work on the sly. Jesse bends down to pick up a bag, and when he gets back up, Walt's there, with his swatter weapon. This is all being presented like a horror movie, with Walt and his fly-swatter made of tape standing in for Jason Voorhees or whatever. He slams his hand down on the vat and proclaims there will be no cooking. "That's an order." Jesse reminds him they're 50-50 partners. Walt's retort to that is to WHAP him on the arm a couple times with the swatter. Which sounds a lot more powerful than I gave it credit for. Jesse jumps back and wails about his arm, but Walt alerts him to a more pressing matter: the fly has landed on Walt's forehead. Walt stands completely still, and you know what's coming. Even if this episode weren't already surrealistically comedic, this would be coming. Jesse gets his free shot at Walt, at Walt's own request. He takes the swatter from Walt, lights up like a Christmas tree and pledges to "make it count," then whales the holy hell out of Walt's head. Did he get the fly? Does it matter? Probably not to Jesse. Walt starts looking on the ground for a corpse, as Jesse is exasperated anew that it's still not over. He points to a dark speck on the floor under a table, but that turns out to be a raisin. Don't they all? Jesse says he "definitely got it," but he's soon distracted by Walt's buzzing nemesis. Jesse marvels in half-serious appreciation. "It's got some serious skills, yo."