Unproduced TV story
Nightmare Country was an unproduced TV story written by Steve Gallagher which was meant to be aired as part of Season 21. The story was scrapped because Eric Saward deemed it too expensive to produce.
The story would have featured a virtual simulator, Reality Simulator, which The Doctor would enter in order to help a species called the Engineers. The Engineers had no home planet of their own, and so they survived using their technical expertise.
The Doctor agreed to test the simulator, and knew that there was an built-in escape factor for him, should anything go wrong. However, inside the simulation, The Doctor had no memories, including who he was, where was the escape hatch or how to use it. (DWM 296)
On 14 November 2019, the story was released as an audio adventure by Big Finish Productions, written by the original writer Steve Gallagher.