Gilbertus prepares a risky lesson in the Mentat School's Discussion Chamber while fifteen of his best students are already being trained for Manford Torondo's crusade. Troubled by the anti-machine cause but unable to state his opposition openly, Gilbertus decides to teach objectivity by having students debate whether restricted thinking machines might safely benefit humanity.

Gilbertus selects Alys Carroll, a fervent Butlerian student, and secretly lies about a coin toss so that Alys must argue in favor of thinking machines. Alys is horrified and refuses, insisting that machines are anathema and forfeiting rather than defending them. To continue the exercise, Gilbertus switches roles and takes the Machine Apologist position himself while Alys defends the Butlerian view.

The debate grows dangerous as Gilbertus argues that not all technology should be condemned because of Omnius, and that machines could perform agriculture, construction, medicine, and navigation without enslaving humans. He challenges Butlerian assumptions about the atomic purges and points out that human slavery has replaced useful machinery, making the moral argument more complicated. Alys falters, but Butlerian students become angrier as Gilbertus's reasoning undermines their convictions.

The classroom turns hostile. Students interrupt, shout disagreements, and some leave, with one accusing Gilbertus of being a Machine Apologist. Gilbertus realizes he may be winning the logic of the argument but losing control of the audience and exposing his true sympathies.

Afterward, Gilbertus locks himself in his office and retrieves Erasmus's hidden memory core. Gilbertus admits he may have made a grave mistake, while Erasmus observes that the Butlerian students wanted confirmation rather than debate. Gilbertus fears reports will reach Manford when Manford returns from the Tlulaxa worlds, but Erasmus treats the crisis as an intriguing example of how quickly events can go wrong.