Aha, 'tis my first Doctor Who review for this blog! Rather than a review of every single aspect, this will be a general look and a kind of 'why you should watch it' thing (more for Who beginners). A quick bit of background: Planet of Evil, by Louis Marks (writer of a Dalek episode and everything) was shown in 1975, and it was the second story of 'Season 13'. It's also the first story that sees the 4th Doctor/Sarah Jane on their own, they'd previously been with UNIT Member Harry Sullivan.
For "Planet of Evil", the 'why you should watch it' part is really quite easy. You should watch it because it is undeniably Doctor Who. It's almost a stereotypical Doctor Who episode, it epitomises everything that it is Doctor Who. The general checklist for classic episodes is: do the Doctor and companion get separated and/or blamed for crimes? Is there a human colony somewhere? Is there a scientific concept that very loosely plays on real science? Admittedly the last question is more flimsy for Planet of Evil, but the other two questions are ticked. I normally find the whole Doctor/companion blamed thing really boring, and it happens a fair bit during the 26 years of the classic series. Planet of Evil does it in the traditional way, complete with human soldiers who are as arrogant and annoying as they should be. But for once, the arrogance isn't as annoying, and the annoyance is therefore less present. Yeah, so the Commander of the soldiers goes over the top - but the thing is there are those who don't go over the top to calm him down, to argue with him, and you'll like them a lot more because they're telling the over the top Commander he's wrong.
The actual monster itself, anti-matter, is one that's popped up before. The Three Doctors is a good episode and uses the idea of anti-matter a lot - and so in many ways Planet of Evil should recycle the same ideas. But it doesn't. It takes a relatively simple idea, threads it through and makes it seem more complicated than it is. The effect of the 'pure energy creature' is done really quite wonderfully, and the separation of a certain Hyde character (you'll know who I mean when I get to Part Four) is a really good effect. The set is stunning (and everyone will say that about this story). The planet looks amazing, the direction on it is amazing, and while things look a bit less good when you get to the spaceship, it makes it feel all the more Doctor Who-y.
Now for the negatives. (otherwise I'm in danger of sounding like Robert Shearman backing up 'The Space Museum' (discreet Whovian joke)). It's slow. There isn't much of a plot, and you feel that you're missing something when you watch it because it is uneventful. It's just the same plot line, reused, shifted round a bit, and running after the other. There's no overlap, there's only one storyline, and it a little too simple, a little too focused. If it expanded to create another plot (perhaps the planet was becoming more and more absorbed by anti-matter too so there was another anti-matter infection in danger of spreading around), then the four parts would feel justified. Though it may have felt slightly squashed as a two-part, it would have been incredibly action-packed and there'd be a sense of urgency. Taking two parts off and squashing the story would be a challenge, but I think that perhaps this story would be appreciated for plot and set and direction, rather than just the latter two.
So it is a good story, and I would recommend it for.. not a new Whovian, it's probably too slow. They should watch Genesis of the Daleks, or The Caves of Androzani, or The Five Doctors (or buy the Revisitations 3 box set and watch all of them, 'cos they work in that capacity), but if you're used to Doctor Who (and it's occasionally varying standards, pace and tone), then this would be good to launch you into the rest. It wouldn't be a favourite - but it should certainly be admired.