Planet of the Spiders

Planet of the Spiders was the fifth and final serial of season 11 of Doctor Who. It was the final regular appearance of Jon Pertwee as the Third Doctorand introduced Tom Baker as the Fourth Doctor in the final moments of part six. This was also Richard Franklin's final appearance as Mike Yates, outside of cameos.

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 Whomythos.

Counting K'anpo's regeneration along with the Third Doctor's, this is one of two of the Doctor's regeneration stories (the other being Twice Upon a Time) to feature a case of double regenerations and the only televised story where more than one Time Lord regenerates.

Synopsis
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.