M is for... Mestor

In the legends of the planet Jaconda, an ancient ruler insulted the Sun God, who sent down a plague of giant slugs to devastate the land. The deity eventually relented and sent a great drought, which destroyed the creatures. It later transpired that this was no mere legend, however. The giant slugs were a parasitic alien race known as Gastropods. One of their eggs had survived and hatched out, and soon the creatures had swarmed over the planet once again to lay it waste. They were led by a Gastropod with enhanced mental abilities - Mestor. He could read peoples' minds and even place his own mind within them. He could kill by thought - a favoured mode of execution being death by embolism, when air bubbles would appear in the victim's bloodstream.

The current ruler of Jaconda was a Time Lord named Azmael, who had broken away from his own people. To save his people he was forced to co-operate with a scheme of Mestor's to create new sources of food. A pair of smaller outer worlds were to be shifted into a closer orbit to the sun, so that they might be used for agriculture. They would be shifted in time as well as as space. Azmael was sent to Earth to abduct a pair of twins - Romulus and Remus Sylvest - who had the mathematical prowess to carry out this task.

After discovering that Mestor had a vast store of eggs which were practically indestructible, but responded to high temperatures, the Doctor realised that Mestor had a secret plan to destroy the planet. The two smaller worlds would plunge into the sun and wipe out the planetary system - sending the activated eggs hurtling through space to colonise other planets.

The Doctor and Azmael joined forces to defeat Mestor. The older Time Lord allowed him to take over his body whilst the Doctor destroyed his physical form with a bottle of acid. Azmael was at the end of his final incarnation and allowed himself to die - leaving Mestor to expire without a host body to inhabit.

Played by: Edwin Richfield. Appearances: The Twin Dilemma (1984).

• Richfield had previously played the Brigadier-surrogate Captain Hart in The Sea Devils.