The Light (1963: The Assassination Games)

The Light were a species believed to be the secret rulers of the universe, similar to the Illuminati in Earth conspiracy theories. They applied their genes in prenatal drugs of other species to take over the unborn, and existed as a thousand species across a thousand worlds through time and space. Ace compared them to cuckoos. (AUDIO:  1963: The Assassination Games) Aside from Light members proper, they also used Light Sleepers, sleeper agents that were activated in case of emergency. (AUDIO: Rise and Shine)

They arrived on Earth in 1850 and, seeing the British Empire as the greatest power in the world, took a foothold. However, by 1963, the United States of America and the Soviet Union had become the superpowers, and the Light had not infiltrated them. They developed the Starfire warhead project as a means of destroying the US, who would be expecting an attack from the Soviets during the Cold War. The Seventh Doctor helped to defeat them, but acknowledged that they were real and would want revenge. (AUDIO: 1963: The Assassination Games)

The Sleepers soon attempted to destroy the Earth and make it appear like humanity had done it, to cover the Light's tracks. (AUDIO: Rise and Shine)

William Heaton MP claimed to be the last member of the Light left on Earth after the Starfire incident. (AUDIO: The Keep) He assembled Counter-Measures to combat the Sleepers (AUDIO: Rise and Shine) but afterwards, it became obvious that the Light had more survivors and was attempting to increase its influence on Earth again. Counter-Measures learned how to detect Light members and killed Heaton, but Toby Kinsella then disavowed and betrayed the team to save himself from the rest of the group. (AUDIO: Clean Sweep)