So with Jesse gone, and Walt obsessing over data, suddenly we're flitting around the back of Walt's head. Walt finally notices the fly when it grazes his ear, then settles upon the piece of paper he was looking it. Walt sizes it up, gets half a grin on his face in anticipation, and slams his hand down on the paper, all the way to the table, where it makes a loud CLANG. After a second, he lifts his hand, but there's no fly under it, dead or otherwise. Walt looks around and sees it on the workstation behind him. He turns back to his work, like he's going to fool the thing, and then round back on it, slamming his clipboard down with an even louder CLANG. Spoiler: everything in this lab, including Walt's clipboard, appears to be made of stainless steel, so this whole hour's worth of fly-swatting is very loud. Anyway, Walt misses again, so now he's on a hunt. With clipboard in hand, he stalks around the lab, swiping at the air, occasionally hitting precious lab equipment.
Already pretty frustrated, he sees the fly settle on the ceiling, well out of his reach. He looks at his clipboard like he's going to throw it at the fly, but he soon thinks better of it. That's good. I was worried Walt had really lost it just th-- ...oh. He's going to throw his shoe at it instead. Okay. After a couple unsuccessful attempts, Walt launches one that actually busts a light fixture (leaving one particularly gnarly chunk of light bulb hanging around on the lab floor). Oh, also, the shoe is now stuck in said fixture. So there's that. Walt climbs the stairs with a broom, but the shoe is just out of his reach. So he does the INCREDIBLY smart thing and steps out to the other side of the guard rail and starts swinging. He does actually knock the shoe down this way. But when he looks across the rail, he sees his sworn enemy, the fly, just hanging out. So he does the exact thing you're hoping he doesn't do: he takes a big swing at it with the broom, throwing his whole balance off, falling from the top walkway, BOUNCING OFF ONE OF THE VATS, and landing hard on the floor. I seriously thought he landed on that hunk of glass and severed his spine or something. Either way, he grounded for the moment. Which is when the fly decides to land on his glasses. That's a dick move, fly, but I have to respect it.
After the break, it's the next morning, and Jesse shows up for work. He extinguishes a cig and notices one of the butts in his ash tray has lipstick on it. Which: I realize Jesse's a slob, but I kind of don't buy that he wouldn't have dumped out his car ash tray since before Jane died. It would have to be pretty overflowing at this point, right? Anyway, Jesse gets all sad-faced.