For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
We meet the four characters on the outskirts of the penal colony, preparing for the execution of the condemned man. The officer gives the background on the machine, and manages to catch the explorer's interest. The explorer learns how the condemned man was "judged," and begins to become uneasy with the colony's way of doing justice.
Act II
The more the explorer hears, the more uneasy he becomes. The condemned man is put in the machine, making the explorer feel as if he needs to reach a decision. Finally he's put on the spot when the officer tells him about the decline of the procedure and asks for his help. The explorer tells the officer no.
Act III
The officer realizes that it's all over, frees the condemned man, and decides to put himself in the machine. But everything goes wrong, and he's killed quickly and nastily. The explorer, greatly affected, sees the grave of the commandant and leaves the colony with haste.