Synopsis: Sometimes the best thing for a time cop like the Doctor to do is . . . nothing. Investigating a "nexus point" in the prehistory of Earth, the Doctor and Ace find some human hunters from the future who've come back to find an early mammal. Ace wants to stop the hunt, fearing the consequences, but the Doctor, instead, stops her.
The hunters thus kill the mammal, which immediately makes intelligent, time-travelling dinosaurs appear on scene. By killing off the mammal, it seems, dinosaurs were allowed to survive and become the dominant form of life on Earth. But when they then turn on the human hunters--still somehow alive--and kill them, both dinosaurs and humans disappear back into the nexus.
History, the Doctor muses, has corrected itself, because the humans were killed before they had a chance to cause the damage to the timeline.
Notes: The Doctor's explanation doesn't really hold up to scrutiny, because, of course, the act of killing the early mammals should have made the human hunters be replaced by the dinosaur hunters. How, exactly, the humans and dinosaur hunters could have co-existed is not sufficiently explained.