There's Something On Your Back
Mike Yates is a disgraced UNIT officer, but when he senses trouble in a meditation center he has joined he calls in his friend Sarah Jane Smith. He hopes if there truly is something sinister going on, she'll be able to contact UNIT and the Doctor. Mike assumes the Brigadier is less likely to believe him since he pulled a gun on him last time they met. (See 'Invasion of the Dinosaurs').
Meanwhile, Jo Grant has written the Doctor and has sent back the blue Metebelis crystal he gave her as a wedding present. Still traveling the Amazon with her husband Cliff, the natives believe the crystal to be bad luck. Indeed, the sentient giant spiders of Metebelis have been looking for the crystal and will stop at nothing to get it back since the Doctor 'stole it'. (See 'the Green Death').
When the Doctor and Sarah are drawn to Metebelis 3, they meet the Spider Queen who is desperately seeking the blue crystal to give to the 'Great One'. Interestingly, the voice of the Spider Queen is Kismet Delgado, wife of Roger Delgado (the Master). The Great One wishes to increase her mind even more than it is. On Metebelis, the Spiders are the dominant species, though the Doctor finds help in the humans who wish to overthrow the Spiders rule.
After escaping, the Doctor and Sarah search for the blue crystal to keep it safe from the Great One. Their search leads them to Abbott K'Anpo Rimpoche, of the meditation center. But K'Anpo senses something on Sarah's back and helps the Doctor see she picked up a spider while on Metebelis. The spider is controlling her and new viewers of Doctor Who might remember 'Something On Your Back' of Donna, which this reminds us of. (See 'Turn Left' of the new Doctor Who series).
K'Anpo turns out to be another Timelord from Gallifrey who ran away to seek a quieter life on Earth. The Doctor receives the blue crystal once more and K'Anpo forces him to realize what he must do with it, return it to the Great One, something the Doctor actually fears. As the Doctor escapes, K'Anpo is killed by men who also have Spiders on their backs, but he is able to regenerate. Turns out K'Anpo was the same hermit who advised the Doctor back on Gallifrey. (See 'The Time Monster')
Crystal in hand, the Doctor returns to the Great One to give her the crystal she craves. She wishes to use it to energize a giant crystal web, to increase her mind and intelligence to infinity. But the Great One doesn't realize the power she has created and her mind quickly burns away. Despite the deadly energies in the chamber of the Great One, the Doctor manages to make it back to the Tardis and UNIT headquarters. But his body is beyond repair and he regenerates in front of Sarah and the Brigadier.
WHO: The Doctor
DEPARTURE LOCATION: Earth, circa 1974
REASON FOR DEPARTURE: His body is damaged beyond repair in the chamber of the Great One.
NEXT: The Third Doctor In Review.