Deep Time

The Fourth Doctor faces a new threat in the form of the Cemmenians, a race of temporal conquerors. They have decided to add Earth to their temporal empire . While the Doctor is racing to warn Earth,the TARDIS, pursued closely by a Cemmenian time cruiser, is struck by one of their powerful weapons, which hurtles the TARDIS millions of years into the past. There, the Doctor must try to fix the injured TARDIS , deal with warring factions of Silurians, and somehow return to the present to prevent his adoptive planet from conquest.