The Ice Warriors

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

The TARDIS arrives on Earth at the time of a new ice age and the travellers make their way into a base where scientists commanded by Leader Clent are using an ioniser device to combat the advance of a glacier.

A giant humanoid creature, termed an Ice Warrior by one of the scientists, has been found buried in the glacier nearby. When thawed, it revives and is revealed to be Varga, captain of a Martian spacecraft that landed on Earth centuries ago and is still in the glacier. Varga sets about freeing his comrades and formulating a plan to conquer the Earth - Mars itself now being dead.

The scientists meanwhile realise that continued use of the ioniser could cause the alien ship's engines to explode. Their trusted computer is unable to advise them without further information, and it seems that disaster is imminent. The disaffected scientist Penley, supported by the Doctor, eventually decides to risk activating the ioniser. There is only a minor explosion, which destroys the Martians and, at the same time, checks the ice flow.

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