But when the capsule returns to Earth it is found to be empty. As the Doctor and Liz investigate, they discover that the interior of the capsule is highly radioactive: if anyone was inside they would now surely be dead.

Have the astronauts indeed returned to Earth? And if not, who are the sinister space-suited figures who stalk the countryside and whose very touch means instant death? 
 

Seven months after leaving Mars, the Probe 7 ship has not returned to Earth. And the rescue ship returns - poisoned with radiation and apparently abandoned. Whatever piloted it back to Earth is not human ...

The Doctor must negotiate with an alien intelligence for the return of its ambassadors - or face the destruction of the Earth ...

The Ambassadors of Death, featuring Jon Pertwee as the Doctor, was written by David Whitaker and broadcast in 1970. This adaptation is by Terrance Dicks who was script editor of Doctor Who for five years and who has written more than sixty novelisations of the television stories.

Doctor Who - The Ambassadors of Death has recently been broadcast on BSB television.