A humanoid race, Kleptons were distinguished by their green skin, huge eyes, rubbery snouts and large, finlike ears. They stood about three feet tall. The First Doctor and John referred to them as bug-eyed monsters. (COMIC: The Klepton Parasites)
Some time before the 30th century, the Klepton home world began moving closer to its sun, becoming hotter and drier. In search of a new home, they learned of Earth and brought a spaceship crewed by two human astronauts to their world. The humans refused to help the Kleptons plan an invasion and were imprisoned. Shortly afterwards, the First Doctor, John and Gillian arrived on the planet and soon were also captured by the Kleptons. The travellers helped the astronauts escape and flee the planet. (COMIC: Prisoners of the Kleptons)
Millions of Kleptons were killed when their planet moved closer to the sun. Some escaped to a world inhabited by Thains and built an underwater city. In the 30th century, they attacked the Thains' city with their flying machines and destructive creepers, capturing many as slaves. The Doctor, John and Gillian arrived and helped the Thains repel the invaders. They followed the Kleptons to their city, freed their slaves and destroyed the city. (COMIC: The Klepton Parasites)
By the 40th century, the Kleptons still hated the Thains, even though they had died out ten thousand years earlier. They disliked humans because they reminded them of the Thains.
In 3999, a gang of Kleptons attacked Sam Jones and Kyle Dale on Micawber's World, having mistaken her for a Thain. The fight was broken up by Space Security Agents, who arrested the Kleptons. (PROSE: Placebo Effect)