The Doctor escapes a web cocoon in this episode because he once knew Harry Houndini.
You know, he also knew Jo Grant, and escaping was her one big talent. Just sayin’.
Yes, the Doctor, Sarah Jane, and some older native to the planet are trapped in cocoons. At some point, a spider will eat them. When will that happen? Eh, eventually. But then a guard comes to take Sarah Jane away and the Doctor worms his way out of what should be sticky spider-webs.
Fortunately, the Spider Queen doesn’t want to eat Sarah Jane. She asks Sarah for help getting that darn crystal back. Sarah agrees under some conditions that I don’t trust a giant spider enough to actually follow through on.
As for the Doctor, he meets the Great One, an unseen spider that outranks the Queen. She does good impressions, and she nearly makes mashed potatoes of the Doctor’s brain. He eventually staggers off, finds Sarah Jane, and doesn’t seem too concerned when she teleports him back to the TARDIS.
Oh, as for the older native guy, his adult sons rescue him with the special headgear the Doctor showed them how to make. The only person in trouble with giant spiders now is the power-mad Lupton.
Back on Earth, we have other problems. Captain Yates tried to chant Sarah Jane and Lupton back, but the spiders snuck behind him. They manage to knock out Yates and the assistant abbot before taking control of Lupton’s cronies. That would be about when the TARDIS pops up in the middle of the room. Fortunately, the now-genius Tommy shows up to save Sarah Jane and the Doctor from deadly energy blasts. He’s somewhat immune. Tommy gave the crystal to the abbot, so he takes the Doctor and Sarah Jane to see that old man. His name is K’anpo.
The Doctor thinks he seems familiar. K’anpo seems to know the Doctor. This is Doctor Who. These sorts of things happen all the time.
As it is, Tommy is keeping Lupton’s spider-enhanced goons outside the room. That may not work much longer.
The next episode is Pertwee’s last as the Third Doctor, not counting a special appearance here and there. Let’s see how the Man of Action Doctor goes out.