It's in this episode that sees Scorby and The Doctor form an uneasy alliance against the Krynoid, and, by extension, an increasingly mad Chase. The scenes between the two in the darkened cottage are completely gripping, and John Challis and Tom Baker raise their games even higher than the lofty acting heights they've achieved this far. They're equally matched by the wonderful Elisabeth Sladen.
Sladen's nearing the end of her time in Doctor Who, and it is tough to find a better performance by her in the series. The way she pushes Scorby's buttons later in this episode is brilliant, and her response to Scorby's chauvinistic remarks is just lovely as she quickly smiles, then blasts back before she bolts out the door to be heroic: "What was that you just said about women?"
And then there's Tony Beckley as Harrison Chase. His being taken over by the Krynoid beast in this episode is a truly haunting scene, made even more effective by Geoffrey Burgon's evocative score. His rallying speech to his plants is just as terrifying. This story has either lucked out by securing so many great actors in so many great performances, or been fortunate enough to have Douglas Camfield as director to choose the actors in the first place.
The UNIT years officially wane down to an anticlimax when the replacements for the Brigadier and Benton (Major Beresford and Sgt. Henderson, respectively) arrive on the scene. I'm glad that two fill-in characters were used, as it would have been sad to see the Brig so underused, and it would have been even sadder to see Benton be garden food.