A is for... Ambassadors

... of Death! An unnamed alien race, from an unspecified world, who sent ambassadors to Earth in the first year of the Doctor's exile to this planet. Though not native to Mars, they had landed there and encountered the crew of the Mars Probe 6 craft. Their bodies were saturated with high levels of radiation, upon which they thrived, and they did not realise that this would be fatal to humans. One of the two man crew of the Probe ship died after exposure to them, and the survivor - Carrington - escaped back to Earth harbouring a fear and hatred for the aliens. This developed into an obsession in him, and he became determined to expose them as killers and to destroy them. The aliens planned to send ambassadors to Earth by abducting the crew of Mars Probe 7 and the pilot of the Recovery 7 mission sent to discover what had happened to the abductees. The three human astronauts would have been returned unharmed once diplomatic moves had been initiated. Carrington arranged for the aliens themselves to be abducted by forces loyal to him, in his guise as the new head of security for the UK space mission programme. They were manipulated into carrying out a series of crimes - thefts and murders. Carrington then planned to unmask them in the middle of a global live news broadcast, inciting the humans to attack the aliens.

The Doctor made a solo space flight to the Recovery 7 craft, and was taken aboard the alien spaceship - a vast glowing structure that appeared organic rather than manufactured. He met the captain - at least on screen - and agreed to retrieve the ambassadors before the aliens retaliated against the Earth. Once Carrington's plan had been foiled, the Doctor left Liz Shaw to help mission controller Ralph Cornish establish diplomatic ties with the aliens. Presumably the creatures decided that the time was not right to make their existence known to the human race - intent to return at another time, as no more is ever heard of this encounter.
Throughout their stay on the Earth, the ambassadors retained the borrowed spacesuits of the humans they had abducted. One is seen briefly without its helmet. They are blue skinned humanoids, but with distorted features.

Played by Ric Felgate, Steve Peters, Neville Simons. Voiced by Peter Halliday. Appearances: The Ambassadors of Death (1970).

• Peter Halliday had just provided all the voices for the Silurians in the preceding story, and was called upon to voice the alien captain in this. The astronaut versions of the ambassadors only speak towards the very end of the story, when the Doctor perfects a communications device.
• The actors who play the human astronauts also play their alien counterparts hidden within the spacesuits.
• Ric Felgate appears in the first three of director Michael Ferguson's Doctor Who stories. He was Ferguson's brother-in-law.
• This design of spacesuit proves durable, as one of the colonists on Uxarieus some four centuries later is seen to be wearing one.