"Planet of Evil" is one of those routinely underrated great stories of Doctor Who, somehow having been overwhelmed by the adventures surrounding it. Really, it's quite a good story: basically, it's the DW version of the film "Forbidden Planet", with some nice Dr Jekyll and Mr Hyde material thrown in towards the end.
Tom Baker and Lis Sladen are on their usual par (beginning a long run of popular stories with the Doctor and Sarah travelling together, without Harry), and David Maloney once again provides solid direction. The guest stars again stand out - most notably Frederick Jaeger - and there's nothing much to fault! Even the effects are pretty good for the time, although at times they are still rather negligible.
If you've tried the more well-known Tom Baker adventures from the period, such as "Genesis of the Daleks" or "Pyramids of Mars", give this one a whirl - you might enjoy it!