The Ambassadors of Death

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

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.

The Doctor makes a solo flight in Recovery 7 and docks with Mars Probe 7, still orbiting in space. He is then intercepted by a huge alien spaceship and taken on board, where he finds the real astronauts unharmed. The aliens' Captain threatens to destroy the Earth unless their three ambassadors are released.

The Doctor is allowed to go and, after returning to Space Control, discovers that the kidnapping of the ambassadors is part of a scheme devised by xenophobic ex-astronaut General Carrington to discredit the aliens and convince the world's authorities to wage war against them. The Doctor and UNIT are able to thwart his plans and arrange the safe exchange of ambassadors for astronauts.

5 out of 107 (4.7%) raters say this story requires a previous story.