The Mind Robber was a a story of the original Doctor Who, encompassing five episodes. It was the second story in the sixths season of Doctor Who, and starred Patrick Troughton as The Second Doctor, Frazer Hines and Hamish Wilson as Jamie McCrimmon, and Wendy Padbury as Zoe Heriot.
After the events of The Dominators, and the ensuing volcanic eruption, the Doctor hits a "quick getaway switch" to protect the TARDIS from a stream of lava. As is almost always the case when the Doctor hits a random switch, the TARDIS does something unexpected: it flies off beyond space and time and enters a realm of imagination. He gets separated from his companions, and they run around a dreamscape populated by such figures as Lemuel Gulliver, Blackbeard the Pirate and Cyrano de Bergerac. The rulers of this realm want someone of great intellect and creativity to help them continue their fantasia, and the Doctor is an obvious choice. However, the Doctor and his companions manage to outwit him, and disappear, with the entire thing possibly being nothing but a dream.
This episode is an early entry into the realm of metafiction, and is a playful and fun, if somewhat nonsensical, adventure in the land of whimsy. As an example of the type of thing that happens in this episode, when the actor who played Jamie McCrimmon became ill with chicken pox, it was written into the story that the Doctor had to reconstruct his face on a cardboard cutout, and failing to do so properly, got a Jamie with a different face, played for one episode by Hamish Wilson.
This episode could have been a "serious" look into fiction and the construction of reality. And if you look closely enough, you might see that. And you might see it years and years later: The God Complex, over 40 years later, is a darker take on this same idea. But in this instance, it comes across more as a children's show, a light-hearted adventure. This is especially interesting in the context of the direction the program was going: the next story after this would be The Invasion, a story showing Doctor Who as a spy-fi show in the mold of The Avengers. "The Mind Robbers" seems to be a last gasp of whimsy before the show took a turn for the more serious.