Easily the best thing about this episode, and perhaps the story as a whole, is The Doctor's cold dismissal of Trist as the latter is being led away by security guards for drug smuggling. His quiet "Go away." is just brilliant - steely, yet almost hiding a seething anger that it almost seems as if The Doctor wants Trist gone not because of what the zoologist did, but what The Doctor would do to Trist (and how much he would regret it) if he remained in the room with him.
Tom Baker's stellar performance in that scene is tragically countered by one of his less successful attempts at humour just a couple minutes earlier - an overdubbed comedy reaction to getting beaten up by a gaggle of Mandrels ("Ooh! My arms! My legs! My everything!"). It's a bad, jokey sequence that sticks out like a sore thumb in a story that desperately wanted itself to be taken seriously. It's a weak section of a story that also desperately wanted to be good.
And, for the most part, Nightmare of Eden is a good story. Even the oft-repeated tale of the troublesome production of this story doesn't make it onto screen. There's only a couple of directorial slipups on the part of Alan Bromly, who was just one day from retirement when he was working on this story, but they are minor, and don't detract from the story. Above all, Nightmare of Eden is probably the most successful attempt at hard drama in an era where such an approach was not necessarily frowned upon, but certainly not keenly encouraged.