"The Time Warrior" represents a rare trip back into the Earth's past for the Third Doctor; historical serials were more common in the time of his two predecessors. It does, however, open in the present. Several scientists have disappeared from a top secret scientific research facility and UNIT are asked to investigate. The Doctor discovers that they have been kidnapped by a Sontaran warrior named Linx who is forcing them to repair his spaceship which has crashed on Earth. The problem is that the crash occurred during the Middle Ages; the resourceful Linx has been able to project himself forward several hundred years to the twentieth century, kidnap the scientists and then travel back in time with them. (There seems to be an inconsistency here with later serials. In "The Two Doctors" from the Colin Baker era of the eighties we learn that the Sontarans do not possess the secret of time travel, although they are desperate to acquire it; the Time Lords are equally desperate to prevent them from acquiring it).
Time travel, of course, is no problem to the Doctor, who is soon on the way to mediaeval England in his TARDIS. When he gets there he discovers two things. The first is that Linx has entered into an alliance of convenience with a ruthless warlord named Irongron. Their bargain is that Linx will supply Irongron with "magic weapons" (i.e. guns) which he will be able to use against his enemies in return for sheltering the crashed spaceship in his castle until it can be repaired. The second is that a young journalist called Sarah Jane Smith has followed him back to the Middle Ages by stowing away on board the TARDIS. Besides rescuing the captured scientists, the Doctor has to persuade Irongron's enemy, Lord Edward, to take action before the balance of history can be upset by the introduction of into guns the Middle Ages and Irongron can make himself king. He also needs to explain to Sarah Jane what has happened and to win her over to his side; she initially suspects him of being behind the disappearance of the scientists.
This serial was the first in the eleventh season of "Doctor Who". It also marked a number of other firsts for the programme. It was the first in which the name of the Doctor's home planet is given as "Gallifrey". It also introduced another race of enemies for him to fight against in the shape of the Sontarans. Like the Daleks and the Cybermen the Sontarans are an aggressive warrior race, but they probably have most in common with the Ice Warriors in that both place a high value upon courage and personal honour. (Personal honour would probably be a meaningless concept to a Dalek). Unlike those races, however, they are not seeking to conquer the Earth, as they are involved in a perpetual state of war against a people known as the Rutans.
This serial also marked the debut of Elisabeth Sladen as Sarah Jane, who would serve as the Doctor's companion throughout the remainder of Jon Pertwee's term of office and well into the Tom Baker era. She was probably the first of the Doctor's companions I fell in love with- I was a teenager in the seventies and the good looks of the lovely young ladies who featured in the programme was a good reason to continue watching it. I found the independent-minded Sarah Jane a big improvement on her predecessor Jo Grant who always struck me as a bit of a bimbo.
"Doctor Who" serials tended to vary in tone. Some- the ones that had children cowering behind the sofa- could be seriously frightening. Others, of which this is one, were much lighter; there is a good deal of sly humour in the relationship between the blustering Irongron and the cunning Linx, or the "warrior from beyond the stars", as Irongron thinks of him, and in the way in which the mediaeval characters interpret their visitors from the future. (The Doctor is proclaimed by Lord Edward to be a "great sorcerer"). There is also a lot of humour in the relationship between the Doctor and the initially suspicious Sarah Jane. Even the appearance of the Sontaran Linx (like a mouldy potato, if that's not an insult to mouldy potatoes) seems to have been designed for laughs rather than to frighten. "The Time Warrior" is not the tensest of the Third Doctor's adventures, but it is one of the most enjoyable.