4A2 - Robot 2

It's remarkable how quickly Tom Baker settles himself into the role of The Doctor. Only Patrick Troughton's Doctor took less screen time to "become himself", so to speak, but that short amount of time was filled with confusion in The Doctor's mind and puzzlement on the part of Ben and Polly.

Oddly, for having his regeneration witnessed by two humans, the change over from the Third Doctor to the Fourth is quite smooth. The Brigadier, while mentioning that he had never seen the process before, is fully expectant of what is to come. He is unfazed by the wildly abnormal events occurring before his eyes. Sarah's been given the lowdown on regeneration by The Doctor himself, and then, almost as proof, K'anpo regenerates into a new body (that of Cho-Je) right before her eyes. The result is a thoroughly undramatic transition, allowing for the story proper to develop at a normal rate without having to allow too much of the post-regeneration trauma that viewers have grown accustomed to since Robot.

Of course, could the mind of Tom Baker's Doctor be more addled during any possible post-regenerative trauma than it already is? Baker is marvelous throughout this episode, be it during his conversation with a rambling (and even more eccentric) Professor Kettlewell, calmly interrogating Hilda Winters while casually uncovering clues as to the robot's whereabouts, or doing his best to outwit the menacing eponymous robot at the end. Even this early on in his tenure, Tom Baker is in complete control of any situation he finds himself in.