Raldonn was an alien that crashed on late 20th century Earth. His spaceship was powered by psychic energy and required at least two beings to provide enough to run it. Raldonn's co-pilot was killed in the crash, so he was stranded.
The British government asked him to "help" them with technological research. He was put in charge of Operation Proteus, which he was secretly using to create a virus to mutate a human into his species, allowing him to finally leave the planet.