Exterminate, exterminate... oops, sorry, wrong character. It's just that, with the Daleks so instantly recognizable thanks to their catchphrase, how come the Cybermen have to remain silent the whole time? It's always sort of bugged me, even though those metallic creatures are scarily effective without the aid of words. Why did I make this digression? Because The Invasion: Episode 6 is when the aforementioned villains (Cybermen, not Daleks) make their proper entrance, having been reduced to cameos in parts 4 and 5.
Chapter five, in particular, ended in a most foreboding way, as Jamie, having descended into the sewers to photograph the extraterrestrial threat and help convince the authorities it isn't some kind of joke, was attacked by the supposedly unstoppable Cybermen. He manages to escape (he had to, since he is the Doctor's sidekick) and joins the rest of the team, ready to hear the Time Lord's summation of what is going on. Meanwhile, Vaughn realizes his allies might not intend to honor their part of the deal they struck with him, and decides to prepare the necessary countermeasures.
The multiple conspiracy gimmick brings an increase in pace and tension, thanks to which Episode 6 improves a great deal on previous chapters: with the Cybermen's presence unveiled, there is something real to fear, and the scene where they march on the streets of London is more than enough to make us overlook certain things that tend to weaken the plot (Troughton's function being limited to mere exposition of facts, for example). As in the rest of The Invasion, the best performance is given by Kevin Stoney, who really enjoys the various layers of evil the Vaughn character allows him to exploit.
Overall, a pretty solid episode. Not essential Doctor Who, but quite close.