Anyway, so what we see is Walt constructing a fake ricin dose out of table salt and placing it inside another cigarette, to match the original (which Walt has in a plastic baggie from Saul last week). He then takes the original cigarette, removes the vial of actual ricin, tapes it to the inside of an electrical socket plate in what appears to be his bedroom, and screws it back into the wall. So let's all tip our hats to Anton Chekhov and wait for that nasty little surprise to re-assert itself in the narrative down the road. [Note: And he flushes the original, now-ricin-free cigarette down the toilet, thus getting a sewage alligator hooked on smoking. The nerve. -- Rachel.]
Next, it's off to Jesse's house, where he and Walt montage the fuck out of a top-to-bottom search for this wayward cigarette. Like... inside the stuffing of pillows. I don't even know, you guys. Tip of the hat to Walt for the patience to go through with such a meticulous charade, just so he can, at the end of a long afternoon, point to the whirring, crawling Roomba and suggest that Jesse see if maybe it picked up the cigarette they're looking for.
Surprise, surprise, it does, and when Jesse finally has the thing in his hand, he completely breaks down. The fear, the guilt, the relief, it's all pouring out of him. Mostly the guilt. His own carelessness could well have been the cause of Brock's near-death, even if it wasn't. Whatever relief he might have felt last season when he learned it wasn't ricin poisoning is gone again. It might as well have been. And on top of that, he nearly killed Walt over his jumping to conclusions. Of course, WE know better. We know his conclusions were 100 percent correct. And now Jesse's CRYING? And feeling SAD? And Walt's trying to make him feel better by saying how they "have each other's back"?? Hold me back, you guys, because I am about ready to tear Walter White's throat out of his body. Look what you're doing to Jesse, you MONSTER!
So the part about having each other's backs (why I oughta...), says Walt, is something they should keep in mind, "as we go forward." Jesse's head kind of perks up at that, pulling him out of his shame spiral for the moment. What exactly does Mr. White mean by that? See, Jesse, lacking the self-awareness that he's on a TV show, doesn't realize there are still 15 episodes to go and thus re-starting the meth business is pretty much a foregone conclusion. He's still under the impression that "getting out with our lives" is a possibility.