‘Planet of the Spiders‘ was the fifth and final story of season 11. It was the final regular appearance of Jon Pertwee as the Third Doctor and the first of Tom Baker (uncredited) as the Fourth Doctor.
The story was the first to use the term “regeneration” to describe the biological process that caused Time Lords to change in physical appearance and introduced one of the Doctor’s mentors, K’anpo Rimpoche, as a means to further explain the mechanic and cement it into the Doctor Who mythos.
In part 6, the Doctor realises his greed for knowledge and his theft of the blue crystal from Metebelis III has set events into motion and that he must face his fear and probable death by returning it. He pilots the TARDIS to Metebelis III and enters the cave to confront the Great One, an enormous spider, who desires the crystal to complete a crystalline web that will amplify her psychic power to infinity. The Doctor obliges, but not before warning her that she is doomed to failure. With the crystal in place, she briefly succeeds, before the web overloads and destroys both her and the Eight Legs. The Doctor receives a lethal dose of radiation from the cave and barely manages to escape to the TARDIS.
Watch watch what happens next on the BBCClassicDoctorWho YouTube Channel.