131 Terror on Terra part one

In the 24th century, a great war between the United Federation of Planets and the Zylon Empire has been raging. Each side takes massive casualties, but still the enemy continues to press on. One side has superiour skill, the other superiour number. All it would take is a single event to tip the scales against humanity and allow the insect race to conquer all.
That event is known as the Gamma Effect.

Now, the Zylon Empire has come to the 20th century in an effort to change history and prevent the Federation from ever being formed…one way or another.

Earth’s only hope is a small band of Star Cruisers sent by Starfleet to the 20th century. Can they and the Doctor protect history and prevent the Zylons from destroying the Earth? Or is history about to be rewritten and the human race transformed into mead for the Zylon’s table?