Here we find the doctor and companions Tegan and Adric on a planet where members Earth survey team are beginning to disappear. The leader is convinced the natives of the planet,known as the Kinda are non threatening primitives. After Hindle,a mentally unbalanced member of the survey team captures the rest of the survey team,two captured Kinda along with the doctor and Adric it's realized the Kinda are not in fact primitives. In the meantime Tegan is psychically drawn into the world of the villainous snake-like entity known as the Mara,an evil being that resides in one's subconscious and manipulates events to it's favor. Using the Kinda's advanced telepathy and understanding of the unseen world the doctor is forced into a position of handling the situation.
This probably amounts to being the most cerebral of Doctor Who stories every written. There are many references to Eastern philosophies/meditations such a Buddism and Hinduism as well as a heavy dose of the Aboriginal Austrailians "dream time". This is also one of the classic Doctor Who stories that is so adult,I'd not recommend it for young children. In the character of Hindle especially. There's references to mental illness,perhaps schizophrenia,and even child abuse more directly on the part of the character. Not only that the metaphysical aspect to the story leaves a lot of loose ends open that sort of requires a more thoroughly informed consciousness. Definitely an excellent example of the wondering,sometimes cerebral nature of the fifth doctor.