The Seeds of Death

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

The TARDIS brings the time travellers to Earth in the 21st Century, where they learn that human society is now reliant on T-Mat - a matter transmitting device that beams people and freight instantly to destinations all around the globe. The system, overseen by a Commander Radnor and his assistant Gia Kelly, is currently malfunctioning and the travellers agree to pilot an obsolete rocket, designed by an old-timer, Professor Daniel Eldred, to the Moon relay station to investigate.

They find the place overrun by Ice Warriors, who are preparing for an invasion attempt. To weaken Earth's resistance, the Warriors are using the T-Mat to send Martian seed pods to selected points on the planet's surface. These emit a fungus that draws oxygen from the surrounding atmosphere, making it lethal to humans but ideal for the Martians themselves.

The travellers manage to use the T-Mat to get back to Earth, where the Doctor discovers that the only thing effective in destroying the pods is water. At the local weather control bureau, having disposed of a Warrior left on guard there, he adjusts the instruments so as to cause a downpour, thus ending the threat of the pods.

He then returns to the Moon where, by a ruse, he is able to misdirect the Martian invasion fleet into orbit around the Sun, where it will be destroyed. The remaining Warriors are all killed.

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