While handing the official the $10,000 check to go back in time to hunt for a dinosaur, Eckels gets nervous about the trip and questions if he'll live.
Travis, Lesperance, Billings, Kramer, and Eckels all went back in time going through days and nights over and over until they reached their destination.
When they all reach the T-Rex, Eckels freaks out and runs off the path he was supposed to stay on to get back to the time machine while the others stay and fire at it to kill it.
After Eckels flees, he has to go and take the bullets out of the T-Rex that he left the others to kill. Travis wanted to leave Eckels there, but the others decided not to.
When returning to the present, everything changes such as words, the president, and everyone feels altered when they return. Upon the return, Eckels discovers he stepped on a butterfly that caused the altering. Eckels gets shot on the spot by Travis for altering the present day.
The theme for this story would be don't mess with the past or the future.