In season four Jesse went to Saul's to get his money? Not sure of the exact reason, but Huel frisked him before letting him in the office. At this point Jesse has the ricin cigarette on him. Huel took the ricin cigarette pack and switched it with a pack missing a cigarette.
When Brock showed symptoms of ricin poisoning, Jesse assumed Walt did it because they were the only ones who knew about the ricin. Jesse threatens to kill Walt, Walt denies poisoning Brock and says Gus did it. Jesse believes Walt and helps him kill Gus.
Now back to 5.3B, Jesse is waiting for the "guy" and realizes his weed has been replaced with cigarettes. Huel bumped into Jesse just as he did in season 4. Jesse connects the dots and realizes that Huel took the ricin cigarette, Walt is and has been lying to him, and that it was not Gus that poisoned Brock. He threatens Saul, Saul squeals on Walt.
This post was edited on 8/25 at 9:47 pm