The Enterprise is thrown 65 million years into Earth's past, where Pike lets a Martian ship destroy a planet to preserve history.
A space-time anomaly sends the Enterprise about 65 million years into the past, to a prehistoric Earth. There the crew learns that a Martian ship intends to destroy the fifth planet of the Sol system, and Pike decides to let history unfold rather than interfere.
The result, as the episode shows, is the creation of the asteroid belt, with its consequences for life on Earth and the transformation of Mars into the barren world later known to humanity. The title is the name of Mars's enormous canyon system, and the story is the opening chapter of the fourth season.