The plot concerns an apparently deserted space station which contain cryogenically frozen humans in a suspended animation which has lated tens of thousands of years. There is significant evidence that there is something else being there too eventually being identified as the Wirrin who intende to consume the frozen humans and colonise the earth.
While the plot is certainly interesting, it does seem to me that it is not the crucial aspect to this adventure. It seems to be more setting the scene from which the new Doctor can emerge. The thrust of the story is to establish Tom Baker as the Doctor. In many people's eyes he was to become The Doctor, but that was to be much later.
This Doctor appears younger, relatively than any of his forebears. His dress, although not the more formal dree or morning coat of the first two was not so dandyish as th the third. His clothing too, with more browns and muted colours indicated a warmer, touchier almost fuzzier type of person than the others. Although he certainly had a more serious side, he did not have the sense of gravitas that the earlier Doctors clearly had. His physical appearance, particularly his height, was more powerful but the respect from others more often than not had to be earned.
Throughout this adventure the Doctor was seen to be distracted by novel and interesting things and developments which stimulated his intellectual curiosity while at the same time causing him to lose sight of events closer to home. Yet at the same time his apparent lack of interest in events could often be mistaken for lack of concern when in reality he chose to think while apparently resting. His quick wit and sudden changes epitomised his flamboyant nature and mercurial mind.
This character was thrown into sharp relief especially during this adventure with the comparison made with the latest companion, Harry. His military training and medical mind contrast sharply with the Doctor heightening the perception of intelectual prowess. Thes use of the companions as a contrasting device is also useful at exploring the softer, one could almost say human, aspect of the Doctor's character. With Sarah Jane the Doctor is seen as a much more understanding and compassionate person than in previous incarnations. In a sense this increases the perception of his own humanity, a feature which is further exposed in the later dealings with the Gallifreyians.
All in all a great fourt Doctor adventure and one well worth having.
This was an excellent episode for so many reasons. The set design convincingly detailed the claustrophobic insides of the Ark. An excellent script and great acting obviates your having to know the complicated continuity of Dr. Who (the Doctor, an expatriate member of a race of long-lived Time Lords, travels across the space/time continuum righting wrongs, saving the cosmos and occasionally picking up or losing various strangers who join the cast; stories are told in multi-part serials of about a half-hour each, but the serials themselves are linked; the Doctor's advanced physiology allows him to "regenerate" whenever he's mortally wounded, becoming a totally new man - this Doctor is numero quatro), and for a change, the alien menace are completely new and not Daleks again. And of course we've got Sarah Jane Smith. The pacing is excellent, the suspense unending and the cheesy production values shine. If you're going to get one Dr. Who tape, this is the one to get.