L is for... Light

Light was a powerful alien entity, capable of taking on any form. It set out to catalogue every single lifeform in the cosmos. On arriving on a planet, it would capture examples of the dominant native species - such as a Neanderthal man on prehistoric Earth. Two crew members accompanied Light. One would set out and survey the region, whilst the other would remain in Light's ship to act as an experimental control. The Survey unit would interact with the environment and other creatures and so change and adapt - and the changes could then be measured against the Control.

After its arrival on Earth, Light went into hibernation in central Africa - being glimpsed only occasionally by natives and explorers. One such was the Victorian adventurer Redvers Fenn-Cooper. 

The spaceship could travel at the speed of thought and was transferred to the basement of a house in Perivale, to the west of London. The Survey unit killed the owner and assumed the identity of a Victorian gentleman named Josiah Smith. In order to evolve fully, he planned to use Fenn-Cooper to assassinate the Queen and assume power over the Empire. He kept Control locked up in the ship.

She escaped and freed Light. It took the form of a man in silvery robes. He began to investigate the inhabitants of the house - killing them in his experiments to understand how they worked. 

He soon discovered that life had not only evolved, but it was continuing to evolve, even as he observed it. This began to drive him insane. The Doctor gave the process a push by telling him about fictitious creatures such as dragons, basilisks, 'bandersnatches' and 'slithy toves' which he had failed to catalogue. Light threatened to wipe out all life on Earth with a fire storm to put a stop to evolution, but self-destructed and dissipated before he could do so.

Played by: John Hallam. Appearances: Ghost Light (1989).

• Hallam's time on the production was extremely limited as he was making The Chronicles of Narnia: Prince Caspian and the Voyage of the Dawn Trader for the BBC at the same time.
• He had many genre performances, including King Vultan's lieutenant in Flash Gordon and roles in Lifeforce and Dragonslayer.
• Light's costume was based on artwork by the visionary poet and artist William Blake. The back of the robes looked like an insect carapace, to tie in with the evolutionary theme. Writer Marc Platt had hoped that the character would have wings.