The Prisoner of Peladon was one of the oldest legends on Peladon, passed down to King Peladon by his grandfather. According to King Peladon, an ageless spirit was kept behind a door with four locks and guarded by Aggedor himself. King Peladon said, "as long as the prisoner was kept under lock and key, Peladon would be safe, but if he were released, the kingdom would fall".
Alpha Centauri, knowing that the Pels would be too busy concerned over Federation separatists and refugees from New Mars to pay attention to this superstition, locked Princess Lixgaar of New Mars in the tower the Prisoner was kept in. According to the Third Doctor, this effectively made the legend of the Prisoner of Peladon "real". (AUDIO: The Prisoner of Peladon)