"Spearhead from Space" introduces Liz Shaw as a companion, a woman scientist who doesn't look or act like a nerd. Instead, she's sceptical, sarcastic, and intelligent. She makes a great foil for the Brigadier, who became a regular in the Pertwee years, along with Sgt. Benton and Captain Mike Yeats (Yeats doesn't come along until later). Liz and the Doctor are a natural team, brainy and inquisitive and resentful that U.N.I.T. has essentially forced them into its service. I think that might be one reason why Liz is eventually replaced with Jo Grant; since Jo wasn't a scientist, she could ask the Doctor the questions that the audience needed to have answered.
Our first sight of the Doctor is him falling face-first out of the TARDIS; at the end of "The War Games," the previous episode, the Time Lords forcibly regenerated him from Patrick Troughton into Pertwee. The camera shots conceal Pertwee's face for as long as possible, either having his face out of shot, obscured by his arm, etc.. He's found and brought to a hospital, where his various differences from human biology are revealed: two hearts, a very slow pulse, the ability to put himself into a coma, etc.. At the same time, The Brigadier is dealing with strange things that have fallen from the sky. The Brigadier suspects the man with odd physiology is the Doctor despite his different appearance, particularly after the Doctor recognizes him, but is forced to wait until the Doctor recovers enough to help him.
That part is the most amusing. The Doctor is considerably befuddled by his regeneration, and won't give coherent answers to his physician. My favorite line is when he demands, "Shoes. Shoes!" Later, he escapes the hospital by sneaking into the "doctors only" showers and subsequently stealing clothing and a car. That scene is notable for the only topless Doctor scene until Christopher Eccleston comes along, though the effect is diminished by Pertwee's jaunty shower cap. No comment is made about the tattoo on Pertwee's forearm; not sure if we're supposed to ignore it or not.
The baddies of the ep, the Autons, are quite creepy, but not nearly as creepy as the scenes showing a doll factory in action, and not nearly as weird to me as seeing the world of 1969-1970.
More on the episode.