Pertwee's introduction as the Third Doctor is one of the few offered us without an on-screen regeneration (even the Doctor Who TV movie had a regeneration). People who watched the last episode of the Second Doctor know exactly what happened, but for the Third Doctor it's a mystery, his slate has been wiped clean. Over the course of the story he gets some of it back, but watching him slowly working out the bits and pieces and being surprised when the TARDIS doesn't work provide some really great moments.
Plus, you know, the idea of living plastic and mannequins coming alive with guns literally inside their hands and the desire to kill everyone can make for some interesting television. The Nestene Consciousness' notion here of creating plastic figures of world leaders so they can take over is a pretty diabolical plot. It, along with the introduction of a new Doctor make these episodes exceedingly watchable.
The usual sorts of great extras have been included on this DVD release. Chief among these are a 22 minute making of piece with several individuals (including a Pertwee interview from 1994) from the series giving us a rundown of how the story itself came together and how the new Doctor came in to things. There is another separate piece focused entirely on the show moving from black and white to color. It isn't quite as compelling as the overarching behind the scenes piece, but it is still quite good. A spoof UNIT recruitment film is also present. Other supplementals include audio commentary tracks, a photo gallery, and the usual assortment of PDF materials.
For its introduction of an iconic series villain, for it being the first colorized story for the series, for it introducing us to a new Doctor, for it greatly restricting the Doctor's ability to travel and thereby dramatically changing things, for it just plain being a good story, "Spearhead from Space" is an iconic moment in Doctor Who. More than 40 years after it originally aired in early 1970, it remains a hugely important and fun tale.