An Earth-like planet towards the very outer reaches of the Universe, where a Colony ship fleeing the final destruction of Earth had been forced to crash.
The colonists were trying to survive there. However, under the ground lived the Tractators, who went from planet to planet, burrowing out their mineral wealth.
The Tractators were killing the colonists. The Doctor was able to defeat the Tractators by separating them from the Gravis, their leader, who mentally controlled them all and had caused the ship to crash. The Gravis was deposited alone on the deserted planet Kolkokron, leaving the colonists on Frontios to build a new life.
This is the only story which the TARDIS seems completely destroyed in this episode - as only the Doctors Hat Stand remains.
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The Doctor finds the TARDIS destroyed in Frontios