We're in a sandy valley (the proverbial "barren landscape") on the outskirts of a penal colony. Gathered near a rather unusual machine, "the apparatus," stand four characters with very specific names: "the officer," "the explorer," "the soldier," and "the condemned man." The explorer is a visitor to the colony, and has been sent by the Commandant to watch the execution of the condemned man, which will be performed with the apparatus. The officer, who's quite fond of the machine, eagerly describes it to the explorer.
The apparatus, invented by the former Commandant of the colony, is what the colony's justice system uses to punish people, in a rather unique way. It "writes" a commandment (in very complicated, illegible script) on the body of the condemned man, the very one he's supposedly guilty of violating, and it does this by slowly rotating his body and cutting into it repeatedly with lots and lots of needles. The punishment is meant to "enlighten" the prisoner – as the machine works on him, he comes to learn what commandment he violated by feeling it on his body. The prisoner is somehow transformed greatly by this process.
One other thing: the prisoner doesn't know what his sentence (or the commandment he violated) is beforehand because in this penal colony he gets no trial or opportunity to defend himself. In fact, if accused by someone, it's just assumed he's guilty. This prisoner – "the condemned man" – for instance, was reported by his captain for being insubordinate, and without trial or even being questioned, he was put in chains. He'll have "HONOR THY SUPERIORS" written on his body by the apparatus. The officer finds this form of punishment exquisite.
Having described the apparatus to the explorer, the officer has the condemned man put in the machine. Meanwhile, the explorer has decided that the apparatus and the whole "judicial procedure" of the colony really bothers him. He wonders if there's any way he can put a stop to it. He's in luck. The officer tells him the popularity of the procedure has fallen greatly since the old Commandant's days, when it used to be the most important thing in the life of the colony. Now the officer is its only real defender.
The new Commandant does not like the "procedure," and is hoping the explorer will disapprove of it, too. The explorer is apparently an important guest from a more "enlightened" part of the world, so if he doesn't like the procedure, the new Commandant will seize the opportunity to take action against the officer. The officer hopes (and appears to believe) that the explorer is in favor of the procedure and will use his influence to defend it and save the legacy of the old Commandant.
The explorer feels a bit sorry for the officer, but tells him he can't defend the procedure in good conscience. Actually, he'll speak against the procedure. Resigned, the officer lets the condemned man go. There's nothing left to do but go into the apparatus himself. His sentence? "BE JUST!"
After setting everything in order, the officer gets into the machine and turns it on, as the soldier and the newly freed condemned man strap him in. But something goes horribly wrong with the apparatus, which begins to fall apart. Instead of writing the officer's sentence slowly into his flesh over the course of twelve hours, it simply impales him and makes a bloody mess of his body. He's dead within a couple of minutes.
Afterwards, the explorer goes with the other two to visits the teahouse in the colony where who the old Commandant lies buried. His grave is hidden, unceremoniously, under one of the teahouse tables, bearing an inscription that predicts he'll return to lead his followers to triumph. Everybody in the teahouse just laughs at it. The explorer prepares to leave the colony in haste, getting in a ferry to return to his steamer. The condemned man and the soldier try to follow him, but he keeps them from jumping into his boat.