Doctor Who: The Ambassadors of Death (TV)

The Doctor joins UNIT's investigation into the mystery surrounding Mars Probe 7. Space Control, headed by Professor Ralph Cornish, has had no contact with the astronauts on board since it started back from Mars seven months ago, and now the Recovery 7 rescue mission has run into similar difficulties. This second ship does get back to Earth, but the astronauts are kidnapped after landing and Liz Shaw notices that the Geiger counter is at maximum. It transpires that the ship's occupants were not the human astronauts after all but a trio of radiation-dependent alien ambassadors who had swapped places with them.