MORE THAN SEVEN MONTHS HAVE PASSED SINCE Mars Probe 7 left the surface of the Red Planet, yet nothing has been heard from the astronauts on board. With the stricken vessel now orbiting the Earth, Space Control has dispatched Recovery 7 to investigate.

As astronaut Charles Van Lyden enters the silent ship, there is a deafening electronic blast and his communication ceases abruptly. Back on Earth, the Doctor identifies the disturbance as a message of alien origin and correctly deduces that there will be a reply, which is duly transmitted from a disused warehouse nearby.

As the Doctor and Liz Shaw attempt to decipher the alien communications, Professor Ralph Cornish and his Space Control team successfully bring Recovery 7 back to Earth. However, the craft is empty. It transpires that human conspirators have kidnapped its occupants – three silent, space-suited figures, alien ambassadors whose very touch is fatal to humans. It becomes increasingly apparent that there is a top-level conspiracy involved, prompting the Doctor to pilot Recovery 7 into space in order to locate the missing astronauts.