This story is another in the darker period of Doctor Who. Having jettisoned the increasingly Andy Pandy jump-suited Sarah Jane Smith before an apparently final battle with the Master, the Doctor foregoes his Presidency to return to roaming around the Galaxy.
The plot has been laid out by several reviewers before me and I will not travel that well trodden road any further. Suffice to say that the character of Leela clearly is intended to attract an older audience but at the same time, the darker stories required a much stronger character than the Sarah Jane Smith types in order for the plots to work. Leela is certainly that, a savage killer, brought up to kill or be killed, she is uneducated but not stupid. She has an instinct which saves them more than once and is yet very protective of the Doctor.
Part of the charm of this character was that she served as a role model for many other female actresses in British drama as an independent person, capable of holding her own with men.
The story is the first to acknowledge that time travel can have unintended future consequences which alters the future in a way never thought of. The two tribes are also symbolic of the way our modern society has developed into an environmental side as opposed to a scientific, technological side. The monster, not by accident, is the same as the monster in that classic sci-fi movie, whose name escapes me, but which is created through the scienytist's id when using alien machinery.
A very impressive story to have on video.