The hunters and safari guides travel approximately 60,200,055 years into the past to hunt a ferocious Tyrannosaurus rex, which has already been spotted and identified by a previous safari guide to ensure that they will not be disrupting the natural order of everything.

Eckels is an inexperienced hunter, and the protagonist of the story, who pays ten thousand dollars for the opportunity to kill a prehistoric dinosaur. Travis is the lead safari guide and emphasizes the importance of remaining on the Path at all times.

Travis also explains the butterfly effect (also known as the chaos theory), which states that everything in the universe is connected, and a small change in the system can have a significant impact that can alter the course of human history. Tragically, Eckels becomes terrified when he sees the Tyrannosaurus rex, runs off the Path, steps on a prehistoric butterfly, and unintentionally alters the course of human history.

In "A Sound of Thunder," the safari travelers go back in time sixty million years to the Cretaceous period of Earth's history, when the Tyrannosaurus Rex walked the earth. Bradbury pinpoints this to an exact year, as safari leader Mr. Travis explains when the group arrives at its destination:

“That”—Mr. Travis pointed—“is the jungle of sixty million two thousand and fifty-five years before President Keith.”

Eckels finds wonder in the journey back through time, but also takes it too lightly, as simply another experience to consume, underestimating the difference between the long-ago past and the modern world and unprepared for the immense power of the giant dinosaur he encounters. Bradbury, always superb with imagery, conveys the marvel and otherness of a vastly different time period, long before humans appeared on the earth, and the ghastly terror of the Tyrannosaurus Rex. His point is that we should be careful in how we use the technologies we create.