Based on the short story "A Sound of Thunder" by Ray Bradbury. The was first published in Collier's (June 28, 1952).

Host: Dinosaurs large and small fill my junkyard workroom. This one given to me by a friend 30 years ago. These given as toys to my daughters. And when they didn't play with them, I simply took them back. So with dinosaurs coming into my life, I often wondered, what would happen if I could go back into theirs. Dinosaurs, time machines. Put them together and you have a tale one million years old.

Eckles: Does the safari guarantee I come back alive?
Agent: We guarantee nothing, except the dinosaurs.

Travis: Didn't you write a book... Dinosaurs I Have Known?
Eckles: Mere fantasy.
Travis: Right. You know nothing until today.

Eckles: Every hunter that ever lived would envy us today. Huh. From now on, Africa will seem like Illinois.
Travis: Have you ever visited Hell, Mr. Eckles? Until you visit Hell, don't make comparisons.
Eckles: I'm ready.
Travis: Yes.

Travis: That is the jungle of sixty million, two thousand, and fifty-five years before the moment you stepped into our office, Mr. Eckles. Christ isn't born yet. Moses has not gone to the mountains to talk with God. The Pyramids lie still in the Earth. Hell.

Travis: If we kill a mouse here, all the future family of that mouse is killed, right? And all the families of all the families. You step on a mouse, you annihilate one, a thousand, a million possible mice.
Eckles: Well?
Travis: Well... For want of ten mice, a fox dies. For want of a fox, a lion starves. For want of a lion, infinite billions of life forms cease to exist. A caveman goes hunting, but you, friend, have stepped on all his lions. The caveman starves. From his loins might have sprung ten sons. From these a hundred. Kill this one man, you destroy a race of people, an entire history of life. Europe remains forever a dark forest. Step on a mouse, you leave your print like a grand canyon across eternity. So stay on the path!

Travis: His breathing could vacuum clean the Empire State Building. Oh, I do go on.

Changes from the short story:

* In the original short story, Deutscher is simply elected instead of Keith. In the episode, the alteration is much more substantial. There is no election, and Deutscher commands "The 15th Rule."