Frontios

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The TARDIS is dragged down to the surface of the planet Frontios and apparently destroyed during a meteorite bombardment. The Doctor is forced to help the planet's human colonists - refugees from a doomed future Earth - and eventually discovers that their problems stem from an infestation of Tractators, burrowing insect-like creatures led by the intelligent Gravis.

The Tractators have been using gravitational force to cause regular meteorite bombardments in order to keep the colonists weak so that they can prey on their bodies and use them as components in their mining machines.

Turlough knows of the creatures through a deep seated racial memory from his own planet. He recalls that they can be rendered harmless by separating the Gravis from the rest of the colony. The Doctor achieves this by tricking the Gravis into reassembling the TARDIS around itself.

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