The Hidden Planet, also known as Beyond the Sun was an unused serial in the earliest years of the series. At one stage it was intended to be the fifth serial of Season 1; later it was intended to be the second serial of Season 2 before it was dropped outright. It was written by Malcolm Hulke and would feature a twin-Planet to Earth. The planet would be dominated by women. It was moved to later in the season and replaced by The Red Fort, and later DW: The Dalek Invasion of Earth. However, it then had to be rewritten after the departure of Susan Foreman, and again to accommodate that of Barbara Wright and Ian Chesterton. It was then abandoned when the season came to a close.
The idea of a twin-planet to Earth would be reused in DW: The Tenth Planet. The idea of a female-dominated race would be given to the Drahvins in DW: Galaxy 4.