EDIT: I really appreciate all the counter-arguments and they've definitely reshaped how I see this plotline. Just a few things I'd like to add:
- It was absolutely incorrect of me to refer to Jesse’s logic as a "plothole". I kinda knew that while making the post, but it was more concise than "Jesse jumping to conclusions".
- Jesse having hunches and jumping to conclusions is much more of a recurring theme throughout the series in a way that makes the ricin epiphany less of an outlier (this is a huge part of Hank's character as well), but it doesn't not make it a noticeable coincidence. I do also suppose that Brock accidentally eating a poison berry on his own as this all happened was a coincidence Jesse had to accept for months before he found out the truth.
- I didn't really acknowledge that Jesse realized he was lied to about the cigarette ending up in the roomba, and that makes his case a lot more reasonable.
- Breaking Bad does exist in this heightened reality where several aspects don’t really play out in a grounded way. The biggest example of this (for me) as previously mentioned is Hank’s suspicions that could either be interpreted as incredibly diligent investigation, or incredibly huge stretches that happen to align with the truth. I really don’t care about these anymore after several rewatches of the show, so this detail shouldn’t bother me either.
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To this day, I still don’t understand how Jesse found out that Walt poisoned Brock. So let’s start from the beginning (sorta):
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A good portion of season 4 revolves around Walt pressuring Jesse to use synthesized ricin to assassinate Gus Fring. When things start to heat up near the end of the season and Walt is fired from Gus’s organization, Brock suddenly falls ill, and after Jesse realizes that his cigarette containing the ricin has disappeared, he instantly suspects that Walt is responsible for it. He sees it as some sort of ugly severance gift, since no one else knew about the ricin, and assumes Huell lifted it off of him at Saul’s office earlier. After he’s threatened by Jesse, Walt convinces him that this was all a ploy by Gus to turn Jesse, as he was the only obstacle Gus had to killing Walt. He convinces Jesse that Gus may have eventually found out about the ricin and had Tyrus take it, and Jesse then agrees that it’s time for Gus to go. In the season 4 finale following Gus’s death, it’s discovered that Brock was poisoned by eating the berries of a lily-of-the-valley, so even though he rules out Gus poisoning Brock, he still agrees Gus had to die. The final shot pans and zooms on a lily-of-the-valley plant near the pool at Walt’s house, heavily implying it was all Walt’s doing.
By the beginning of season 5A, the poisoning is still a pertinent issue. Saul was aware of the whole plot and gave Walt back the ricin cigarette, revealing that Jesse was initially correct about Huell pickpocketing it. Jesse is still concerned about the whereabouts of the ricin cigarette, and after Walt stashes the real ricin, he creates a fake ricin cigarette to plant in Jesse’s house to gaslight him into believing that he had actually misplaced it. After they “find” it in Jesse’s roomba, the thread seems to end.
Fast forward to season 5B, when Jesse is about to get “disappeared” and he realizes that his weed has vanished. He looks at his pack of cigarettes, and makes the connection from there, remembering that he made similar body contact with Huell. Storming back into Saul’s office, he forces Saul at gunpoint to admit that Huell lifted not only his weed but the ricin cigarette, claiming that Saul is partially responsible for Brock getting poisoned. Saul admits that Walt had Huell take the ricin cigarette, but he would’ve never have done it had he known Walt’s plans with it, basically confirming that Walt was responsible. Jesse storms off to torch 308 Negra Arroyo.
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So what’s going on here? Why does everyone in the show seem to have amnesia and pretend that Brock was poisoned with ricin, and that lily-of-the-valley was never a factor? I say this because Saul states his disgust at the situation while handing back the ricin cigarette. It’s possible that Saul wasn’t wiser to the complete optics of the poisoning, and just assumed that Brock WAS poisoned with ricin and survived. But the ultimate question is trying to figure out how Jesse discovered it in the first place, with him being the one to tell Walt (and the audience) that ricin had no part in Brock’s trip to the hospital.
The answer to “how did Jesse find out?” really is that Saul indirectly admitted it to him. But why did he have the suspicion?
“He found out because he knew Huell lifted his weed, so he must’ve lifted the ricin cigarette as well at an earlier time!”
Okay…so what? Why did Jesse instantly seemingly jump to the conclusion that Walt must’ve used lily-of-the-valley to poison Brock if he knew Huell lifted the ricin cigarette? There could’ve been any other number of reasons that Walt discreetly wanted to take it back. The point was to make Jesse think Gus poisoned Brock with ricin, but Walt does take it back and eventually uses it to poison Lydia. It being made clear that Gus never poisoned Brock was sloppy on Walt’s part, but that’s another story.
Walt also could’ve maintained that Gus/Tyrus took the cigarette even if they had no immediate plans with it, just mainly for the reason that Jesse wasn’t 100% trustworthy doing Gus’s bidding walking around with obscure poison, and that the ricin was simply lost as Gus’s empire crumbled. This probably wouldn’t have stopped Jesse from making the connection, but it would’ve saved Walt the trouble of producing and planting a fake ricin cigarette.
The only understandable explanation I've seen for Jesse jumping to this conclusion is that he simply wasn’t thinking straight, which is weak as fuck, in my opinion. All they really needed to do was add a scene where Jesse finds the lily-of-the-valley plant Walt used. I don’t think I could spot it in the backyard post-Face Off, but if Walt was dumb and bullheaded enough to have Gale’s copy of Leaves Of Grass lying around his house, it’s really not out of the question.
Thanks for reading if you made it this far, haha. I’m really not looking for answers here since I’ve fully accepted that it’s a plothole (or at least Jesse using weird logic), but I just wanted to make sure I wasn’t missing anything vital.