The Doctor returns to Metebelis 3 where he must face the spiders’ ruler, the Great One.
Sarah Jane is revealed to be under the control of the Queen, which explains how she and the Doctor escaped so easily. With the blue crystal, which Tommy had given to K’anpo, the Doctor is able to free her mind. The Doctor is astounded to discover an old friend; K’anpo is his former mentor, a fellow renegade Time Lord now enjoying peaceful exile on Earth. The attack of Lupton’s followers is repulsed with the help of Tommy, but K’anpo is injured and forced to regenerate by merging with his assistant, Cho Je, who was a projection of K’anpo’s self.
The Doctor realizes his greed for knowledge and his theft of the crystal has set all these 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 blue crystal to complete a crystalline web that will amplify her psychic power to infinity.
However, once the crystal is in place, 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. He eventually arrives at UNIT (after being gone, from their perspective, for three weeks), and collapses on the floor. Regeneration cannot commence in time to save his life and he dies permanently. As Sarah closes his eyes and weeps, K’Anpo/Cho-Je appears and decides to give the Doctor a little “push”. The Doctor starts breathing again and regenerates into his fourth incarnation — much to the surprise of Sarah Jane and the annoyance of the Brigadier. Sarah Jane exclaims, “Look, Brigadier, Look! I think it’s starting!” The resigned Brigadier answers, “Well… Here we go again.” The face of the Third Doctor fades away and reforms. A younger man, with long, curly brown hair, is born from the regeneration- the Fourth Doctor.