Leto receives a message reading: "A column of smoke by day, a pillar of fire by night" (18.2). Whatever that means.
Leto decides to talk with Jessica and let her know why he's been giving her the cold shoulder.
As Leto walks the hallways, he comes across the dead body of Tuek. He hears a mewing sound and investigates. It's Mapes. She says a few cryptic words before she dies.
Leto is hit in the neck with a poisonous dart. Side rant: how come people always hit their targets with poisonous darts? Have you ever tried to shoot one? It's freaking hard.
Anyway, Leto collapses to the floor but remains conscious.
It's Yueh making his move. Finally. Leto realizes that the doctor has sabotaged the power generators. House Atreides is wide open to attack.
Yueh explains to Leto that he doesn't want to kill him. He wants Baron Harkonnen dead, but the Baron will never let Yueh get close to him. That's where Leto comes in.
Yueh is going to insert a fake tooth into Leto's mouth. If he bites down on it, it will release a toxic gas.
When the Baron gets close to Leto to brag over his victory, and he will, Leto is to bite on the tooth and release the gas. It will kill Leto, for sure, but the Baron will also do the shuffle off the mortal coil.
Yueh says Leto must go through with the plan. If he does, Yueh will save the life of Lady Jessica and Paul. It's a good bargain, since Leto was doomed from the start, anyway. Jeez, talk about the hard sell.
Yueh promises to make it appear as if they are dead and send them to live with the Fremen.
As Leto looses consciousness, he can hear Yueh telling him to remember the tooth.