An Ambassador

The United Nations Intelligence Taskforce (UNIT) is tasked in providing security for a British space programme during a time when a recovery capsule, Recovery 7, has been sent to rendezvous with Mars Probe 7, with which radio contact has been lost while orbiting Mars. The Doctor and Liz Shaw are watching this recovery mission, via a live television broadcast, just as Recovery 7 links up with the Mars Probe. Suddenly there is a loud burst of noise which The Doctor recognises as being a message. This prompts The Doctor and Liz to head for the Space Centre.

Soon after they arrive the message is repeated followed by a further burst of noise which The Doctor deduces is a reply. The Doctor is curious as to what was in the message and who sent the reply. The Doctor tries to decode the message but this proves not to be possible when it is discovered that someone has sabotaged the computers. Meanwhile Recovery 7 has returned to Earth and while it is being transported by UNIT it is ambushed and stolen. The Doctor though is able to retrieve the capsule and so bring it finally to the Space Centre. However, after a security alert at the Space Centre, the capsule is opened. But it is found that the astronauts have already been removed. Liz also discovers that the interior of the capsule is highly radioactive.

The Doctor voices his concerns to The Brigadier that he is certain that someone on Earth has interfered with the recovery mission by kidnapping the astronauts. They manage to meet Sir James Quinlan, the Minister for Technology, who explains that he is head of the newly formed Space Security Department, and that the astronauts have been infected with contagious radiation. Because they did not want the public to become panic-stricken he authorised General Carrington, a former astronaut on Mars Probe 6, to capture the astronauts so that they can be cared for.

The TARDIS Console

The Doctor does not believe this and so demands to see the astronauts. General Carrington reluctantly agrees to take them to the lab where they are being held. But before The Doctor arrives a thug called Reegan removes the three astronauts from the lab. When The Doctor realises, from the very high radiation readings, that the astronauts who returned from space are alien and that the human pilots must therefore still be in space.

The alien astronauts have now collapsed while being looked after by a Doctor Lennox. Under orders from General Carrington he reluctantly 'feeds' them more radiation and to his disbelief they start to recover. General Carrington then orders Reegan to deal with The Doctor and Liz and the latter is kidnapped and taken to work with Doctor Lennox. Liz tries to escape but before she can get very far she is caught by General Carrington who is on his way, in his car, to check on the condition of the aliens.

The Brigadier

Despite the obstruction of the authorities, the director of the British space programme, Professor Ralph Cornish, is determined to organise another space flight to Mars to investigate the situation. However, there is insufficient standard rocket fuel to make the trip but a more powerful M3 variant is available but using it would create more G-force than a human astronaut could stand. The Doctor therefore offers to pilot the ship himself.

As he prepares to blast off Reegan tries to sabotage the probe by increasing the feed of M3 variant, but The Doctor survives the attempt on his life and succeeds in piloting the probe and docking it with Mars Probe 7 still orbiting Mars. However, the three human astronauts are not aboard. Then Mars Probe 7 is intercepted by a huge alien spaceship and The Doctor finds himself taken onboard. Inside The Doctor discovers the three original astronauts are unharmed but mentally deluded into believing they are in quarantine. An alien being now reveals itself to The Doctor and explains the humans are being held captive pending the safe return of their Ambassadors. They had been sent to Earth following a treaty between the race and mankind, but the terms of this agreement have now been broken because of the detention of the Ambassadors. The Doctor offers his personal guarantee to help return the Ambassadors to their mother ship and resolve the conflict before a state of war is declared, and is permitted to leave the alien craft and return to Earth.

However, when The Doctor returns to Earth, Reegan sabotages the decontamination tank gassing The Doctor. The Doctor is then taken to the secret base where Liz and the three Ambassadors are being kept. The Doctor discovers that Liz has been forced to assist a scientist called Lennox to build a device to communicate with, and control the aliens, who are then sent on a killer rampage at the Space Centre.

The Doctor is Attacked

It is then that it is revealed that General Carrington is the person responsible for the current situation, and that his actions have been prompted by xenophobia driven by his own encounter with the alien beings when he piloted Mars Probe 6 some years earlier. When his co-pilot, Jim Daniels, was killed by the aliens the General signed a treaty with the aliens to lure three of their number to Earth, where he planned to unveil their real agenda of an alien invasion – despite this not being the case.

The General then reveals that the next phase of his plan is to force the Ambassadors to confess their plot on public television. Leaving The Doctor and Liz working on a new and improved communication device to communicate with the aliens, Carrington departs for the Space Centre, where he aims to unmask the alien Ambassador before the eyes of the world – and then call on the powers of the Earth to unite and blast the spaceship from the skies.

Back at the secret base The Doctor is able to construct a signalling device to alert UNIT. It is not long before The Brigadier and a number of UNIT troops manage to liberate them, killing Reegan in the process. On learning of what The Doctor discovered while in space and General Carrington’s plans they race back to the Space Centre and manage to apprehend General Carrington before he can make his broadcast. He is taken away, protesting he was only following his moral duty. The Doctor then arranges for the Ambassadors to be returned to their own people knowing that this will result in the three human astronauts being returned.