Depending on whether you see the explorer or the officer as the protagonist, you could read this as either Voyage and Return (explorer as protagonist) or a Tragedy (officer as protagonist). Let's start with Voyage and Return.
When we meet the explorer, we're already in the middle of things: he's already "fallen" into the strange new world (arrived at the penal colony) and is already at the site where he's going to witness the execution of a condemned man. But he's rather bored, distracted by the sun, and not terribly interested in the apparatus.
After the officer has already told the explorer a bit about the old Commandant and the three parts of the apparatus, the explorer starts to become interested in it. He then learns from the officer about what the condemned man's "sentence" is and how the colony's judicial procedure works. The explorer finds it severe, but wonders if it might be justified because of the rigid discipline needed for the colony. The officer then tells him a lot more about how the apparatus actually functions.
By this point, the explorer has made up his mind about the penal colony's way of doing justice: he doesn't like it. Pressure is mounting because the officer, having finished explaining, orders the condemned man put into the apparatus. It's against the explorer's principles to intervene, but he feels pushed to do it. After the officer appeals to him, asking for his approval of the apparatus. The explorer learns of his influence, and refuses to help the officer. This leads the officer to decide to put himself into the machine.
The explorer isn't exactly happy that the officer decided to put himself into the machine, but he respects the officer's decision and decides to stay with him to the end. Once things start to go wrong with the machine, however, the explorer becomes "deeply troubled" and feels helpless. When the officer's mangled body is splayed out over the pit, the explorer's look into his face "against his will." Although the awful event itself is over when the explorer visits the old Commandant's grave, that's actually the finishing touch of his terrible experience – it's seeing the grave that propels the explorer to escape.
The explorer's escape is pretty quick – just the last paragraph of the story – but even through the narrator's understated story telling we can sense a certain urgency about it. The explorer is horrified enough by what he's seen that he wants to get out of the colony as fast as he can, and doesn't want anything more to do with the place. That's why he wards off the soldier and the condemned man.
Now let's examine how the plot breaks down if we view the officer as the protagonist.
The officer is feeling very enthusiastic about the execution at the start of the story, and already (we'll learn later) has in place a plan to win over the explorer so that he can save the procedure he's so devoted to.
The officer doesn't really notice that the explorer isn't interested at first, but once he becomes interested, the officer perceives it and eagerly begins to chat him up about the colony's way of sentencing and executing criminals. Things are looking up for him. He doesn't have any idea that the explorer is already beginning to feel "dissatisfied" with what he's hearing.
When the condemned man vomits it's the first obvious thing that goes wrong for the day – the officer is disgusted, and his precious apparatus is dirtied. Perhaps by his frustration, he's prompted to tell the explorer the truth of his situation: that the new Commandant is trying to eliminate the old justice system. He still hasn't a clue that the explorer is already opposed to it and is not going to be won over by the officer's arguments. He struggles to convince the explorer that he (the explorer) has the influence required to help maintain the current justice system.
The officer learns that the explorer won't help him, and he accepting that it's all over. Nothing left for him to do but go into the machine. At least he'll get to experience the excruciating, beautiful torture and the moment of realization (just before it kills him) he's lusted after all his life. His sentence: "BE JUST!" But then things start to go wrong.
As the apparatus goes to pieces, it kills the officer quickly and brutally, denying him his last, greatest hope. It's unclear what causes this, whether lack of maintenance or the impossibility of his own sentence. But both he and the machine to which he was dedicated are gone. The final blow comes after the fact, when the explorer sees the grave of the officer's old Commandant, who's become a joke and has wound up buried in a teahouse. Nothing the officer stood for remains.