"Well, Sergeant? Aren't you going to say that it's bigger on the inside than it is on the outside? Everybody else does."
"It's pretty obvious, isn't it?"
--The Doctor (3) and Sgt. Benton, in "The Three Doctors"
The idea was to figure out a way to have all of the Doctors to date appear in a single episode. "The Three Doctors" was the first time, though in actuality it was Doctors Two and Three, with Doctor One on a screen--William Hartnell was very ill and his scenes were shot separately, though he appears to interact with his successors. The Time Lords are, essentially, under siege and cannot spare anyone to go against the villain, a rogue Time Lord named Omega, so they send the Doctor his other selves to help him. The fun part is watching Two and Three squabble.
"I can see you've been doing the TARDIS up a bit. [pause] I don't like it."
The Doctor (2), in "The Three Doctors"
Something I'd never noticed in "The Three Doctors": the Doctor (2) is wearing a brilliant blue shirt, a shock since I've recently gotten more used to seeing him in black and white.
Why does Doctor 1 seem so much smarter than 2 and 3? The actor is older, but that Doctor is not--hasn't he learned anything as he grows older? Or is it just that 2 and 3 are more distracted by trouble on the ground, since they're in the midst of an attack?
The schtick with Omega is that he lives in antimatter-land and creates a whole planet by willing it into existence. His body, over time, has vanished, but his will keeps him "alive." It's a shocking moment when he wants the Doctor(s) to remove his mask and there's nothing underneath. Except, I noticed this time that when Omega is ranting and throwing his head back, there's a split second when his mask bounces up and you can see the actor's chin with black makeup all over it, and a bit of his mouth above, which was not covered with makeup. Heh.
I still think the extended scene of mental judo between the Doctor and Omega is tedious.
A world created in the midst of antimatter by an ancient rogue Time Lord apparently looks just like a quarry.
"The Five Doctors" had Richard Hurndall playing the role of the First Doctor, with Susan as his companion; Two traveled with the Brigadier; Three with Sarah Jane Smith; and Four with Romana, but he was "trapped" before he reached Gallifrey (actually, they used old footage from "Shada"). Five, at the time, had Turlough and Tegan. The plot involved a Time Lord scooping up various Doctors and dumping them in an ancient part of Gallifrey, the Death Zone, once used for a sort of war games with unwilling participants; they're all battling obstacles (and classic Who monsters,such as a Dalek, Cybermen, and Yetis) trying to reach a central tower. At a loss, the Time Lords bring in The Master to help out, which is so not a good idea.
"We must know what is happening there."
"Did it occur to you to go and look?"
--President Borusa and the Master, in "The Five Doctors"
Five, once he escapes the Death Zone, is trying to find out who the traitor Time Lord might be; his part seems to move more slowly, because he's on his own, without even one of his companions. The High Council of Time Lords are just less interesting.
Two and Three are quite amusing with each other, and Hurndall did a pretty good impression of One, though he's not a carbon copy. I wish Five had been able to interact more with them.
The whole point of these episodes is to celebrate the long history of the show and remind you how much you loved it, and in that I think it succeeds.