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While the Brigadier investigates a loss of contact with Mars Probe 7, the Doctor realises signals coming from the probe are of extra-terrestrial origin.
While Liz distributes the cure to the virus, the Doctor discovers the Silurians now plan to wipe out humanity by destroying the Van Allen Belt.
UNIT retrieve the returned Recovery 7 but then Carrington hijacks it while the Doctor and Liz try to decode the signal he sent.