Here’s the timeline:

• Walt had at one point cooked up an extremely strong poison called Ricin.
• He wanted Jesse to poison Gus with it.
• Jesse hid it in his cigarette pack, waiting for a moment to slip it in Gus’s food but he kept balking.
• Walt asked Saul Goodman to get the poison cig. Goodman enlisted Huell, who has “fingers like sausages,” to sneak it out.
• Brock comes down with a mysterious illness.
• Jesse loves kids, as is evident in lots of other episodes.
• Jesse looks in his cigarettes — the Ricin cigarette is gone!
• He wonders, “Could Brock have gotten it?”
• Walt plays along and suggests it was Gus who did it. Why not? He has a history of harming kids, Walt suggested.
• In reality, Walt had just slipped Brock much less harmful Lily of the Valley.
• So now Jesse understands the need to kill Gus. Walt does most of the work and blows Gus up.
• Brock recovers fine. He’s not all that articulate, so we don’t really know what happened.
• Walt plants the Ricin in Jesse’s vacuum, so Jesse thinks it was all a mistake. “He still had to die, right?” Jesse says of Gus. Walt agrees, of course.
• Time passes. Jesse is about to skip town and lights up a doobie in Saul’s office, angering Saul. Because the coyote won’t allow anyone with marijuana in his car, he has Huell lift the pot off Jesse. The pot had been hidden in Jesse’s cigarette pack.
• Waiting for the coyote, Jesse goes to grab a smoke and realizes his pot is gone. Then he realizes that the Ricin was likely stolen in the same way — that Walt was the one who poisoned Brock, not Gus.
• And mayhem ensues.