An event that is referred to in Joseph Heller's novel Catch-22.
While en route to Pianosa from the United States, the soldiers are given Atabrine, a drug used to combat malaria. Yossarian does not want to take his, so Appleby orders Kraft to order Yossarian to take the medicine.
It is not clear whether or not Appleby outranks either of the other two men, but he does what he does because he has faith in the rules governing his behavior, something Yossarian distinctly lacks. In an example of surreal Catch-22 logic, Appleby takes four times the normal amount of Atabrine, because he wants to be four times as good a pilot as every other pilot.
Late in the story, Appleby apologizes to Yossarian for this and other transgressions. In all, the episode reinforces Yossarian's role as an existential hero, since he exercises his absolute freedom in the face of opposition.