Rhoos was the home planet of the Kaarks. The Kaark Supremo said that it was the eighth planet of the P'Tuu system, but First Doctor said that it was the third of a GO-star, eighty million miles from its primary.
The giant Cyclops Fo crashed on Rhoos and began ravaging the planet, killing many Kaarks and capturing others to make them his playthings. When the First Doctor and his companions arrived on Rhoos, they discovered that the TARDIS had materialised on Fo's arm. However, before they could return to their ship, they were captured by Kaarks led by Ff'ni. They were taken to the Kaark Supremo, who told them of their world's plight. The Doctor went on a mission to retrieve his ship from Fo's stronghold, rescued his companions, and slew Fo. (PROSE: The Playthings of Fo)
On Olleril, Bernice Summerfield told the Celebroid duplicate of the Seventh Doctor that she was reminded of their "bewildering experiences" on Rhoos, which she described as "the planet of volcanoes." When the Doctor agreed with her, this confirmed her suspicions that something was very wrong with him, as Rhoos did not exist. (PROSE: Tragedy Day)