Planet 12 of the Ridion Alliance was a planet belonging to the advanced Ridion Alliance, before it suffered a Seven Hundred Year War which turned most of the planet into a barren wasteland and its population descended into a dark age. Among the locations on the planet were the ruins of the Palace of Hy-Ridion. A Ridion supercomputer, Momus, planned to re-educate the natives by introducing them to famous figures abducted from various points of Earth's history, but was talked out of this plan by the Tenth Doctor and Donna Noble and convinced to go exploring the universe instead. (PROSE: The Lonely Computer)

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