K is for... Krotons

Krotons were creatures composed of the mineral Tellurium. Their spacecraft were of the same material and were known as Dynatropes, and they used these to travel across the galaxies. They were powered by mental energy, and four Krotons were required to energise each. They could not die as such, only cease to function in their bodily form and revert to basic chemical elements - a form of crystalline slurry.

When one of these craft crash-landed on the planet of the humanoid Gonds, destroying two of their number, the Krotons were attacked. They retaliated by killing many Gonds and leaving a large area around the Dynatrope a barren wasteland. It suited them to claim that this region was toxic, as it would keep the Gonds away from them.

Without the third and fourth members of their crew, the Krotons went into hibernation, reducing themselves to their basic chemical elements. They had given the Gonds teaching machines, which the Dynatrope would use to monitor their intelligence level. Each year the two brightest Gonds would be invited into the Dynatrope, supposedly to be transported away to become servants of the Krotons. When the TARDIS landed in the wasteland, however, the Doctor discovered that the Gonds were having their mental energy absorbed, their bodies then being vapourised by acid out of sight of their people.

His companion Zoe took the Gond test and passed, forcing the Doctor to follow suit as she would have to enter the Dynatrope. Together, they provided enough mental energy to resurrect the two remaining Krotons. They knew that the pair could provide the energy needed to replace the destroyed Krotons, and so allow them to escape from the planet. The departure of the Dynatrope would destroy the Gond settlement, however.

Outside their machine the Krotons were effectively blind, and one of them had to remain in the craft to guide the other if it moved beyond their machine. Eventually they struck a deal with a Gond named Eelek to hand the Doctor and Zoe over to them, on the promise they would leave.

Realising that the Krotons had refused to teach the Gonds anything which could be used against them - such as chemistry - the Doctor devised an acid which attacked Tellurium. He used a quantity of this to destroy the Krotons before the Dynatrope could take off.

Played by: Robert La'Bassiere, Miles Northover. Voiced by: Roy Skelton and Patrick Tull. Appearances: The Krotons (1968/9).

• La'Bassiere (who sometimes dropped the apostrophe) appeared in a couple of episodes of Moonbase 3 - the series which Barry Letts and Terrance Dicks hoped would allow them to leave Doctor Who. His real name was Robert Grant, and he asked for his pseudonym to be used for this story. He also appeared as one of the villain's henchmen in the Bond film Live and Let Die.
• Skelton and Tull opted to give the Krotons an Afrikaans accent.
• Apparently there was a Spinal Tap moment when the costumes were being designed, as they were made roughly half the size they ought to be. The lower part of the costume had to be cobbled together using a rubber skirt. The solid part of the body was supposed to reach to the ground. Supposed to look crystalline, they ended up looking robotic. Robert Holmes, their creator, wanted them to be crystal men.
• For many years the myth persisted that the Krotons had been a Blue Peter design-a-monster competition entry. People may have been thinking of the "Aqua-Man" from the 1967 competition, who looks like he's made of cardboard egg-boxes.
• Holmes reused elements of this story in his final completed work - The Mysterious Planet.