The Seeds of Doom is structured, essentially, as a four-part story set in England with a two-part prologue that takes place in the Antarctic. This approach works terribly well, as the pacing of these first two episodes is quickfire without having to worry about sustaining the same story thread for the next four episodes.
We've already seen brief glimpses of the main antagonist, Harrison Chase, and his minions, Scorby and Keeler. We're properly introduced to Scorby and Keeler in this episode. Remember when I reviewed The Ambassadors of Death and The Mind of Evil and mentioned how Reegan (William Dysart) and Mailer (William Marlowe) were two of the more well written and well acted "thugs" in Doctor Who, and how they were the first of a trio of well acted thugs in Who? Well, Scorby is the third, and possibly the best of them all. John Challis brings such a cold menace to the role, and he is simply magnetic in every scene he's in. Challis is one of many actors who give phenomenal performances in this story, and while his is one of the best, he may have to yield the title of top performance to Mark Jones as Keeler.
Jones's Keeler is a top notch bit of acting. Keeler is so paranoid and so edgy, and is in no way comfortable when forced to hold a gun on The Doctor and Sarah. He is a man out of his element throughout this entire episode. He's forced to pair up with the ultra-violent Scorby, he's not at all happy about being transported to the coldest place on Earth, and, especially, as noted before, holding a gun is totally alien to him. Look at the way Jones awkwardly holds the pistol to guard an already bound Doctor and Sarah. His arm is extended to its full length, holding the gun away from himself as much as he is keeping his prisoners under guard.
Tom Baker once again raises his game, showing a previously unseen side of The Doctor (that of pure rage) when screaming after Scorby when the thug takes Sarah hostage. Only two episodes in, and this might just be the best acted story ever in Doctor Who. And I haven't even started to mention Tony Beckley as Harrison Chase...