The blue crystal that the Doctor took from Metebelis III in a previous adventure is desperately sought by the Eight Legs, a race of mutated spiders, as the final element in their plan for universal domination. With help from an old mentor, the Doctor realises the only way to foil the plot is to make the ultimate sacrifice. The Doctor must risk death to return to the cave of the Great One and save the universe. [tardis.wikia.com]
Planet of the Spiders is the fifth and final serial of the 11th season of the British science fiction television series Doctor Who, which was first broadcast in six weekly parts from 4 May to 8 June 1974. It was Jon Pertwee‘s last serial as the Third Doctor, and marks the first, uncredited appearance of Tom Baker as the Fourth Doctor. It marks the last regular appearance of Mike Yates. This serial introduces the term “regeneration”.
Brigadier Lethbridge-Stewart’s future wife, Doris, is mentioned for the first time in Part One. The character herself wouldn’t make a visual appearance until Battlefield in 1989. Future companion Harry Sullivan is mentioned in Episode Two; he would appear in the next serial, Robot. This story marks the second, and final, appearance of the Whomobile which first appeared in Invasion of the Dinosaurs.
The story was to have been titled The Final Game and was originally intended to write out the character of the Master, with the villainous Time Lord sacrificing his life to save the Doctor. Due to the death of actor Roger Delgado, script editor Terrance Dicks abandoned the project in favour of a new story, which eventually evolved into Planet of the Spiders. Kismet Delgado, his widow, was one of the voices for the Spiders. [Wikipedia]
Watch the Third Doctor regenerate on the BBCWorldwide YouTube Channel.