This was a great farewell to the 3rd Doctor. Our very James Bond esque incarnation of the Doctor got some very James Bond action sequences in this serial, particularly in the second episode! Lupton steals the Whomobile, the Brig, Sarah and Benton go after him in Bessie, the Doctor goes after him in a helicopter, a police car joins the chase, Lupton abandons the Whomobile and the chase comes to a halt, then Lupton takes off in the helicopter, then the Doctor and Sarah go after the helicopter in the Whombile... And the Whobile FLIES! Oh and then there's a boat chase, too! This really was full of Pertwee action and goodness.
The Eight Legs are truly unnerving. They've left me feeling a bit cold, actually. They give off a true sense of evil, almost demoniacally. Some say that they're badly voiced but I think it just added to their eeriness. The story itself of the Eight Legs enslaving the humans and trying to gain universal omnipotence was convincing and, like I said, left me feeling quite moved.
The Doctor's death was dignified and honourable. He realised if he hadn't stolen the crystal then the Eight Legs would have destroyed themselves anyway and the lives lost due to them trying to get it back from the Doctor would not have been taken. So, he makes the ultimate sacrifice, and enters the chamber that will both destroy the Eight Legs and the cells of his own body.
He has to "face [his] fear," but it isn't really stated or explained what his fear is. Personally I think it was a fear of allowing evil to win because of him; he feared that the Eight Legs would become all powerful over the universe because of him, so he had to face it and stop it, even if it meant giving up his life. I don't know if there are other theories or an actual definitive answer, but that's what I inferred.
This is a good story and a nice farewell to the 3rd Doctor. It's sad to see him go. Usually we don't see the Doctor die, we see him dying and then regenerate, but this time he actually dies and needs a push in order to regenerate. The fact we actually see the breath leave his body makes it even sadder.
He is a great incarnation. I'm glad I can finally say I've watched all of his run, I've really enjoyed it! Pertwee's Doctor is very down to earth, rational, and he takes no crap. But he's also suave and funny, compassionate, and he's a peace-maker, but if you try to use force against him, he'll respond with the same force in self-defence. Pertwee is easily one of my favourite Doctors. I can't wait to watch my favourite stories of his again, but for now... It's onto Tom!