The story begins in a penal colony on an unnamed tropical island with no stated nationality.  A military Officer shows an apparatus to a man known only as “the Traveler” who was invited by the Commandant to witness an execution. They are in a small valley with a sandy ground, surrounded by rocky cliffs. Two other men, the one condemned to die and a soldier who guards him, stand nearby. The Officer and the bored Traveler sit in chairs next to a small pit across from the apparatus. The Officer explains that his former Commandant, now deceased, invented the apparatus. The Officer has great admiration for the former Commandant, saying that the man created a perfect penal colony, and he proudly describes the three parts of the apparatus. The Bed is a large wooden platform covered in cotton wool. The Designer is a wooden box the same size as the Bed, hanging approximately six feet above it. In between the Designer and the Bed is the Harrow, a steel bar with a tool that shuttles back and forth like a sewing machine with multiple needles.  

The Traveler expresses curiosity as the description continues, and the Officer is encouraged by his interest. He tells the Traveler that when the condemned man is in place and secured, the battery-powered Bed is turned on and begins to vibrate according to specific calculations along with the movements of the Harrow. The Traveler interrupts to ask about the sentencing process, and the Officer is amazed that the current Commandant did not explain it to him already. The Officer says the former Commandant would never have neglected to describe the process to a visitor and seems disgusted by the idea of the current Commandant not proudly explaining everything to the Traveler. However, the Officer restrains himself and continues his explanation, noting that he is the best person to do so since he has the former Commandant’s original plans for the apparatus and states that the former Commandant created the entire design himself. 

The Officer continues his description, explaining that once the apparatus is turned on, the sentence begins. Whatever crime the prisoner has committed is written on his body with the needles of the Harrow. Because the prisoner who’s still standing by does not seem to understand anything the two men are saying, the Traveler asks if he knows what’s going to happen to him. The Officer reveals that not only does the prisoner not know his sentence, but he never went through a trial. At this news, the Traveler finally exhibits some emotion, shocked that the prisoner never had a chance to defend himself against the charges against him, and he stands from his chair. The Officer calms the Traveler, explaining that this is simply how it’s done. The Officer, as the assistant to the former Commandant, is the judge of the penal colony and also its executioner. He says he is guided by the idea of never doubting the guilt of those who are accused. 

The Officer explains the case against the condemned man who was a servant to a captain. The condemned man was supposed to salute the captain’s door every hour. However, he was caught sleeping on the job and subsequently sentenced to death. The Officer says if he’d wasted time interrogating the condemned, the man would just lie, so there was no need to bother. The Officer goes back to explaining the execution process and turns on the machine which creaks loudly. The Officer is embarrassed by the noise but continues explaining how the Harrow is made of glass so observers can watch the needles piercing the condemned man’s body again and again, deeper and deeper. Like a tattoo gun, there are pairs of needles, a long and a short one. The long needle makes the puncture, and the short needle, instead of injecting the ink of a tattoo, sprays water to wash away blood into the pit below the apparatus. 

The condemned man approaches to inspect the machine along with the horrified Traveler. The Officer throws a rock at the soldier to get him to pull the condemned man back before he explains the machinery inside the Designer. He pulls out the plans from the former Commandant, indicating the value of the pages because he refuses to let the Traveler touch them. The Traveler can’t read the language on the pages, and the Officer says it’s a script that must be studied to be understood. Ultimately, the Officer explains, the entire execution takes twelve hours as the needles pierce deeper into the body of the condemned as he’s turned repeatedly to ensure his crime is written on his entire body. The Officer says after six hours, there is a transition at which point the condemned understands his crime and experiences a kind of spiritual enlightenment. At the end of the explanation, the Officer orders the condemned man who’s still watching intently to be stripped and strapped to the Bed. 

The Traveler wants to intervene because he knows what he’s about to witness will be an act of brutal, unjust inhumanity. However, he remembers that the current Commandant not only opposes the execution practice but also the Officer himself. The Traveler decides to fulfill his role as an invited observer. Meanwhile, the condemned man vomits through the filthy rag in his mouth, fouling the machine. The Officer is furious that the Commandant refuses to provide not only a new rag but also the supplies necessary to keep the apparatus in good working condition. He says the current Commandant is unduly influenced by the women he enjoys keeping around him. He admits that although there used to be audiences who gathered to watch executions, he is the only remaining defender of the practice. He says the people who gathered to watch, especially children, benefitted from the moment of transcendence. The Officer is emotional but pulls himself together as the soldier finishes cleaning the vomit. 

The Officer reveals he knows why the current Commandant has invited the Traveler and asks him to defend the execution process rather than condemn it. The Traveler tries to be evasive, telling the Officer that it seems to him that the days of the practice are already ending, but the Officer pleads with him to stand with him against the Commandant and publicly support him and the apparatus. The Traveler refuses, but he does agree to speak with the Commandant privately and not condemn the Officer or the execution process in front of a large audience. The Officer is surprised that his demonstration did not convince the Traveler of its value. He frees the condemned man, ordering the soldier to get him out of the apparatus, and then he pulls out a page of the Commandant’s plans and asks the Traveler to read it. When the Traveler says he’s still unable to decipher the text, the Officer spells out the words, “Be just.” Then he adjusts the Designer while the condemned man retrieves his clothes from the pit. 

The Officer strips off his clothes, gets in the Bed, adjusts the Harrow, puts the filthy rag in his mouth, and moves to turn on the apparatus. The soldier and the condemned man, however, approach to strap the Officer down and start the machine. Instead of creaking, the apparatus moves silently and smoothly across the Officer’s body. The condemned man steps closer to watch, and the Traveler tells him to leave. The condemned man, however, drops to his knees to beg to stay. Just as the Traveler decides to drive him and soldier away, the Designer begins to fall apart. The Traveler steps closer to check on the Officer only to find that the Bed is not turning the man, and the Harrow is stabbing him. Before the Traveler can figure out how to turn off the machine, the Harrow lifts the Officer’s bleeding body, impaled on the needles. The Traveler sees that a large spike went through the Officer’s forehead, killing him quickly and denying him of the redemptive enlightenment he’d admired. 

The Traveler, the soldier, and the condemned man go to a teahouse where the former Commandant is buried. The soldier explains that the man wasn’t permitted to be buried in the churchyard, something the Officer was ashamed of. They show the Traveler the former Commandant’s grave under some tables. The inscription on the headstone predicts the Commandant will return to gather his followers and regain control of the colony. It’s clear everyone in the teahouse thinks the prophecy is ludicrous. The Traveler hands out coins and leaves. The soldier and condemned man follow him, and he assumes they want him to take them with him. By the time they get to the dock, however, the Traveler is already on the boat, and the ferryman has cast off. The Traveler takes a heavy rope and threatens the soldier and the condemned man to keep them from trying to jump aboard.