Episode one is particularly action-packed, with a helicopter and hovercraft providing probably the greatest concentration of real hardware in one episode until Pertwee's swan song 'Planet of the Spiders'. Subsequent episodes are more studio-bound (with some of the most painfully cramped 'outdoor' scenes ever committed to videotape.) But despite that the story fair barrels along without the usual quagmire of capture-escape-recapture that plagues six-parters - partly because of the slightly bizarre left turn it takes around episode 4. (The worst I can say about the pacing is that the Doctor spends too much time sitting on his hands, but given that Troughton is pulling double duties that's understandable). It's a highly melodramatic story, and the late plot twist involving Salamander's buried secret stretches credibility almost to breaking point, but David Whitaker's deft script never loses control of its pulpy twists and turns. Unlike some Who from the era, this holds your attention right to the end.
Troughton's performance as would-be dictator Salamander is broad, particularly the 'interesting' choice of a thick Mexican accent, but he's utterly unlike the Doctor and really shows his versatility. (It's notable having seen Orphan Black that the two Troughton characters don't share the screen until the finale, presumably a by-product of production constraints). In fact Whitaker crafts several strong characters who transcend their various 'types' - notably including an extremely capable female character in Astrid, and a rounded black female character in Fariah - with the help of a mostly excellent main cast.
It all wraps up a tad swiftly and conveniently, hinging on one too many character reversals and convenient coincidences, but not enough to mar a thoroughly enjoyable serial.