The short story “A Sound of Thunder,” by Ray Bradbury, opens when a man named Eckels enters the offices of Time Safari, Inc., a company that offers safaris that take hunters to any time in the past to hunt any animals they wish to shoot. Eckels presents a check for $10,000 to the clerk and asks if the company guarantees that hunters return alive from the past. The clerk replies that the company guarantees nothing but encounters with dinosaurs. Hunters must strictly obey their guides, shooting only what and when they are instructed to shoot. Any disobedience will result in a $10,000 fine plus possible government-imposed penalties.

Eckels contemplates the nearby time machine and remarks that if Keith, the progressive candidate he favored in yesterday’s presidential election, had lost the contest, Eckels might be in the office now, trying to go to some other time to escape the outcome. The clerk agrees that it would indeed have been awful if Deutscher, Keith’s opponent, had been elected. The two men quickly return, however, to discussing the opportunity Eckels will have to shoot a Tyrannosaurus Rex. The clerk warns that if Eckels is attacked and eaten by a dinosaur, the company is not liable. Six safari guides died last year, along with twice as many hunters. The clerk wants Eckels to be sure that he really wants to make the trip.

Eckels indicates that he does, and so he is introduced to Mr. Travis, the experienced guide who will lead this safari. Travis and Eckels, carrying rifles, enter the time machine, which is already occupied by Travis’s assistant, Lesperance, and two other hunters (Billings and Kramer). The machine kicks into operation and the nights, days, weeks, months, years, centuries, and millennia whiz by. Leaving A.D. 2055, the men soon arrive in the midst of a jungle that existed 60,002,055 years before.

As they look out of the time machine, Travis points out a metal path into the jungle. Made of anti-gravity metal, it hovers half a foot above the ground. It was placed there by the company to prevent hunters from in any way having physical contact with the jungle. Travis emphatically instructs them that they are never to leave the path, for any reason. He insists that the men pay careful attention to this rule and never violate it. They are not to touch anything, and they are not to shoot at any animals unless Travis approves. When Eckels asks why, Travis elaborately explains that the company does not want to take any chances by changing anything at all about the future. Destroying even a flower, an insect, a mouse, or any other living thing could cause potentially massive unforeseen consequences in the future. This is especially the case since killing one living thing in the jungle would mean wiping out the potential offspring of that thing, and thus the offsprings’ offspring, and so on and on and on for countless generations. The results of killing anything in the jungle are literally unpredictable, which is one reason that all the men are wearing oxygen helmets—so as not to introduce any latter-day bacteria into the primeval jungle. Killing just one animal might some day mean, for instance, that a future caveman might not survive by eating the distant offspring of that animal, and the non-survival of that caveman could conceivably change human history. In short: no one is to kill anything except an animal designated by the company.

The company, as it turns out, sends scouts back in time through the time machine to the precise location of the hunt, but they arrive before the hunt and observe which animals will accidentally die in any case on the day of the scheduled hunt. Those are the animals—the dinosaurs in particular—whom the hunters are allowed to shoot: the animals whose deaths would not change a thing. The scouts mark these animals by shooting them with red paint so that they can be easily identified. Then, the hunting party arrives, stations itself where the animal was scheduled to die by accident in any case, waits a few minutes for the animal to arrive at that spot, and then tries to kill the animal with guns. Yet shooting at dinosaurs is still a very dangerous business, and survival is not guaranteed.

Right on schedule, a tyrannosaurus doomed to be killed by a falling tree branch within a minute, suddenly appears where the men have been hiding. Huge and ferocious, he also stinks from the stench of his recent meals. He makes a “sound of thunder.” Monstrous, he is much larger and more terrifying than Eckels had ever imagined. Eckels begins to panic, especially when the T-Rex spots the men. Eckels wants to flee, but he is cautioned to move slowly back to the time machine if he is too afraid of the approaching dinosaur, which now dashes forward at lightning speed. With Eckels having fled, the remaining men fire their guns directly at the monster’s head. Surprisingly, it suddenly drops to the ground, making an enormous noise. The men can hear the different parts of its body shutting down in death. The men are covered in the dead beast’s blood. As they clean themselves, a massive tree branch falls on the dinosaur—the branch that would have fallen on and killed him anyway. His death has changed nothing about the future.

When the guides and the other two hunters return to the time machine, they see Eckels. Travis, angry at Eckels for nearly getting them all killed, screams at him and points to Eckles’ feet. In his panic, Eckels had run off the metal path. If nothing else, this means that the company will be financially penalized. Eckels is stunned at Travis’s anger and offers to pay ten times the cost of the trip to make up for his mistake. Instead, Travis orders him to go out onto the path alone and thrust his arms into the monster’s mouth in order to retrieve the bullets, which should not be left behind in the past. A few minutes later, Eckels, with arms bloody up to his elbows, returns to the time machine, carrying bullets. Travis thinks Eckels has now learned a lesson: not to hunt a beast he fears.

As the time machine whizzes the men back into the future, Travis stares contemptuously at Eckels. When the men arrive back in 2055, everything looks the same—almost. Somehow, Eckels senses small differences, such as a strange faint smell in the air. The clerk looks the same, but also somehow different. The most noticeable difference is a sign hanging in the office, which now looks badly misspelled. Eckels is stunned and collapses into a chair. He cannot believe that merely stepping off the trail would have changed anything in the present. Even when he notices a dead butterfly on the bottom of his shoe, he cannot bring himself to accept the idea that its death could have changed anything. Turning to the clerk, he asks who won the presidential election of the day before. The clerk, who had earlier admired Keith and detested Deutscher, now feels exactly the opposite and praises the victory of Deutscher, whom he extols as an “iron man,” a tough, strong leader.

Eckels moans and drops to his knees. He grabs the butterfly and wonders whether there is any way to take it back in time and restore it to life. He hears Travis raise his gun. The last thing Eckels hears is the thunderous sound of a bullet being fired.