I'm honestly not sure whether it's the production team being particularly aggressive in trying to sideline Susan in this episode, or whether Carole Ann Ford is just at the point where she's willing to commit suicide-by-cop in order to get out of doing any more of this story, but she is practically ludicrous in her devotion to being a wet lump of nothing in "A Change of Identity". Having already decided in the previous episode that escaping from the jail cell is far too icky to deal with, she's now giving up on their one-in-a-million free ticket out of having her head chopped off because she's got a bit of a headache. Thankfully, new characters Jules and Jean are here to literally drag her back into the plot.
Apart from Susan's now-total devotion to uselessness, this episode feels like it's really firing on all cylinders. Spooner's style of storytelling finally reaps some real rewards once the jail cell is left behind and characters can get properly embroiled into the plot; Susan and Barbara (but mostly Barbara) get embroiled in a good old-fashioned Pimpernel pastiche, while Ian gets to escape from jail and go trotting off on his English spy subplot. Lemaitre gets to be sinister and suspicious in a way that really should combine with his name to make you think that he's the Master in disguise (oh, come on! I can't be the only one who thinks that!) And the Doctor...
Arguably, this is the moment where the Doctor becomes the Doctor proper. I know that other people will point to 'The Edge of Destruction', where he finally softens towards his companions, or at some point in 'The Daleks' or 'The Sensorites' when he helps save planets for the first time, but Spooner finally adds a key element that's been missing from the character that will come to be indispensable. The scene where the Doctor wanders into a tailor's shop, and through a bit of puckish bluff comes out as a provincial official fully prepared to use his forged credentials to throw his borrowed authority around in order to wrongfoot authority figures who throw their weight around, well...it's the entire Troughton era for a start. It's a tried and tested standard of the series all the way up until the psychic paper makes it virtually effortless, and Hartnell nails it so well on his first try that you can understand exactly why that is. The way his face remains stern while his eyes twinkle with mischief just sells every moment of his deception, both at the tailor and in the prison. There's something so natural about seeing the Doctor bluff his way into the halls of power, all the while giggling at the absurdity of it all, that it really feels like the show has come home.
And of course, bluffing his way into authority carries with it the constant threat of discovery, and embroils himself deeper into events...while bringing him face to face with one of the great architects of events. Yep, that's exactly how the series comes to work from now on. This is where Spooner comes into his own, and where his influence on the series and its protagonist begins to be felt most clearly.