In the original story by Rog Phillips, one character believes he is in a room, and the other character believes he is in a spaceship. They challenge each other's idea of the truth of reality, with fatal consequences. The story was originally published in Astounding magazine (October 1958).
This episode was wiped by the BBC and no copy of it is known to exist.
Later remade as an episode of "Out of the World".
A reconstructed version of the episode using still images combined with an off-air soundtrack recording is featured on the 2014 BFI region two DVD release of the series.