The irony in Ray Bradbury’s 1952 short story A Sound of Thunder lies mainly in the story’s ending. Eckels is a man on a mission: hunt and kill a dinosaur. Towards this end, he arrives at the corporate headquarters of TIME SAFARI, Inc., which promises clients the opportunity to travel back in time for the purpose of killing a now-extinct animal. As the name plate at the company’s door announces:
SAFARIS TO ANY YEAR IN THE PAST.
YOU NAME THE ANIMAL.
WE TAKE YOU THERE. YOU SHOOT IT.
Eckles, however, rapidly presents himself as a potential head-ache for the seemingly put-upon tour guide Travis, whose wariness of yet another expedition into the past is apparent in his demeanor. His inquisitiveness proves increasingly annoying to those around him, and prompts Travis to set forth the philosophical case against journeys such as this. Explaining to Eckles and the other clients making the journey through time that any failure to comply with company rules regarding conduct while on the journey could have very serious ramifications for the future, Travis reveals the extent of his concern about client conduct and about the potential implications of a failure:
"We don't want to change the Future. We don't belong here in the Past. The government doesn't like us here. We have to pay big graft to keep our franchise. A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species."
Eckels remains confused about this philosophical conundrum, which prompts Travis to launch into a protracted hypothetical scenario about what could go wrong should Eckels or any of the other clients wander from the specially-constructed path. Upon hearing the approach of the Tyrannosaurus Rex that the hunters have been guaranteed an opportunity to kill, however, Eckels panics and flees, which brings us to the story’s more ironic elements.
In fleeing back to the time machine, it is revealed that Eckels broke the rules and diverged from the specially-constructed path, trampling nature under foot as he ran in terror. The dead butterfly he has inadvertently but carelessly killed represents precisely the kind of problem to which Travis had referred: The dead butterfly will now never live out its normal life, which can result in a sequence of events at variance with the course of history that would otherwise have evolved and led these men to their encounter at Time Safari, Inc. in the first place. When the group returns to the present, much has, in fact, changed. The presidential election that originally ended with the victory of Eckels’ preferred candidate, Keith, has instead resulted with the other candidate’s election, Deutscher, which we have been led to believe will augur ill for society as a whole (In the words of an employee of Time Safari, Inc., “If Deutscher had gotten in, we'd have the worst kind of dictatorship. There's an anti-everything man for you, a militarist, anti-Christ, anti human, antiintellectual.”)
The first major element of irony, then, involves Eckels’ actions resulting in the exact opposite of what he had wanted, and what had originally occurred. He has, indeed, altered the course of history through his carelessness, which prompts the story’s final element of irony, Travis’s killing of Eckels with his hunting rifle, the firing of which creates “a sound of thunder” -- a sound eerily reminiscent of the sound of thunder heard when the Tyrannosaurus Rex emerged from the forest.