Planetoid 50

Planetoid 50 was, as described by the First Doctor, an obscure body in the Karpellian system that held no life.

When suffering a retro-regeneration crisis, the Master came to Planetoid 50 to try and focus their mind. Reading The War of the Worlds, the Master decided to recreate the novel on Planetoid 50, creating a race of "Martians" and even abducting several inhabitants of 19th century London, including the Paternoster Gang, to populate the simulacrum they'd created.

After the Fighting-Machines had grown beyond the Master's ability to control, and caused them to regenerate into Missy, the Doctor, also suffering from retro-regeneration, arrived on Planetoid 50, being turned into the Tenth Doctor. Meeting up with the Paternoster Gang, the Doctor managed to thwart Missy's scheme, though failing to prevent her escape, before he returned the Gang home and freed the now pacified Fighting-Machines on the surface of Mars. (AUDIO: The Martian Invasion of Planetoid 50)