The increasingly tense power struggle between Salamar and Vishinsky that has been building for the past couple episode finally comes to a head in Episode Four. My favourite moment of this struggle actually occurs during the climax of Episode Three (although it is reprised at the beginning of Episode Four).
It happens when both The Doctor and Sarah are about to be ejected into space, as ordered by Commander Salamar - an order opposed by Vishinsky. The two men fight to gain control of the lever that will eject The Doctor and Sarah into space. However, Salamar isn't fighting Vishinsky to try and push the lever himself. He is fighting Vishinsky in order to try and make him push the lever and, thus, obey his command. The adherence to procedure has been the main thread going through Salamar's character arc. When Salamar is deposed by Vishinsky, Salamar goes mad due to the lack of procedure, steals a nuclear accelerator, and dies trying to destroy the anti-matter creature that Sorenson has become.
I'm glad that, at the end of this story, Sorenson survives instead of falling forever in the antimatter pit on Zeta Minor. It gives the story a sense of redemption, and gives The Doctor a chance to change Sorenson's mind about the supposed source of energy he has found. Had Sorenson not survived (as script editor Robert Holmes originally lobbied for, only to have his mind changed my producer Philip Hinchcliffe), then another scientific expedition might have returned to Zeta Minor, attempted to mine the same energy, and the whole business would have happened again, with disastrous results. This can almost be seen as an allegory for the over-drilling of oil on Earth, and the need to find an alternative source of energy that won't damage humanity or hold them back as the antimatter found by Sorenson did.
Overall, Planet of Evil is an enjoyable story, often overlooked by the heavyweights on either side of it.