This story is the highlight of the Tom Baker era and one of the best Dr.Who stories of them all. The Antarctica scenes are well done and the story never sags across six episodes. The transformation scenes are almost painful to watch and the large Krynoid at the end is surprisingly effective. Scorby (played by John Challis- Boycie from Only Fools and Horses) undergoes genuine character development over the course of the story so that we're almost saddened by his (somewhat gratuitous) demise. Evoking memories of The Thing From Outer Space this was one of the last truly creepy Doctor Who stories - sublime.