A rather self-serving wealthy man, Eckels contracts with Time Safari, Inc. to take him in a time machine in order to have the thrill of shooting the Tyrannosaurus Rex, one of the dinosaurs of the beginnings of the world. This time travel involves returning to a site in which the creature killed died without disturbing the balance of nature.  For instance, the Tyrannosaurus that Eckels will shoot has died from a falling tree, so killing him will have no other effect that his original death, and, thus, not alter the course of time.

What is of utmost importance in this return to a past time is that nothing be disturbed other than the dinosaur to be killed. Therefore, Eckels is given strict instructions to remain upon the company's anti-gravity path so that no dirt, no grass, no insect, etc. is disturbed or picked up on his shoes or person.

A touch of the hand and this burning would, on the instant, beautifully reverse itself.  Eckels remembered the wording in the advertisements to the letter.

Because this time travel is so dangerous and exacting, Eckels must pay a great sum of money in order to travel back in time. If anything happens that is not according to regulations, he will also have to pay another ten thousand dollars.