With the United Nations Intelligence Taskforce providing security, the British space programme under Professor Ralph Cornish oversees the launch of the Recovery Seven probe. This has been sent to Mars to make contact with the missing Mars Probe Seven and its two astronauts, who lost contact with Earth eight months earlier. The pilot of Recovery Seven, Van Lyden, makes contact with the Probe but is then silenced by a piercing unearthly sound. The noise troubles the Doctor who travels with his assistant Liz Shaw to the Space Centre to investigate the situation, offering insights into the origin and meaning of the sound, which he interprets as coded messages. He also identifies a reply message sent from Earth and this is pinpointed to be coming from a warehouse seven miles away. Led by Brigadier Lethbridge-Stewart, UNIT troops attack the warehouse and engage in a gun battle with troops organised by General Carrington.
Meanwhile Recovery Seven has returned to Earth and while UNIT is transporting it more of Carrington’s troops stage an ambush and steal the vessel. The Doctor relocates it, by which time it is empty. Carrington has ensured the contents – three space suited astronauts – are detained elsewhere, feeding them radiation to keep them alive. Carrington is now introduced to the Doctor by Sir James Quinlan, the Minister for Technology, who explains that he is head of the newly formed Space Security Department, and that his actions were to protect the astronauts as they had been infected with contagious radiation. Quinlan states that they did not want the public to become panic-stricken and so Carrington had been acting with authority in his actions.