Robot
Oh, so this is why everyone loves Tom Baker.
I've seen a couple Baker serials before so I wasn't completely oblivious to the Fourth Doctor before going into this. But I was surprised and impressed with how quickly Baker not just stepped into the role, but completely owned it. I had grown to like Pertwee more than I thought I would, but by the end of his run I was feeling burned out. In an instant, though, Baker brings a breath of fresh air to the series, and injects it with a new burst of manic energy and humor. Whereas previous Doctors had a good quip here or there, Baker is consistently and frequently all-out funny -- and not even from just the lines he says, or how he says them, but tiny facial ticks and other body language cues that gets across a lot of personality with seemingly little effort.
This was a passing-of-the-torch story, because it's not only Tom Baker's first outing as the Doctor, but also the last for producer Barry Letts, who had presided over nearly the entirety of Pertwee's tenure. As such, it feels like a pretty standard Third Doctor story: mad scientists unleash evil robot to take over the world, the Doctor and UNIT assemble to stop them. But the routine plot highlights what Baker so quickly brings to the show, making what would've been a tired story feel fresh and fun just by virtue of him being in it. And credit to Terrance Dicks (the outgoing script editor who wrote this episode himself) for just as quickly shifting gears in how he writes the Doctor. I remember Moffat and maybe RTD saying it took them a while to write their Doctors for the actors playing them because you have to become familiar with the actor's personality before you can incorporate it into the role. But right from the start here it's like Dicks has flipped a switch in his brain, writing Baker's Doctor so much more flippant and energetic than he did Pertwee's. Baker's debut probably wouldn't have landed as well as it did if Dicks was still writing him as if he were exactly like the Third Doctor.
I was originally planning to take a month off from classic Who after this, so that I didn't burn myself out. But this was such a funny, energetic second wind for me that I just want to dive back in with Baker's run right away.
