The 2nd Doctor era definitely kicks off with a bang with Troughton already seeming to fit into the role and I really like the humour related to the post-regeneration part of the story as well as the Doctor is already very mischievous (the "Lesterson listen" scene especially). But what stands out to me the most is that the Daleks had never been so menacing up to this point, and Terry Nation didn't even write this story. There's an intensity and bleakness to the whole thing. It's also significant as, with being the first post-regeneration story, there is a mystery to the identity of the Doctor. The story is a slow burn and does have the occasional stagnant moment, but it is easily the most well paced six-parter so far. I especially love the building intensity when the Daleks rise in number and Lesterson suffers a breakdown after he realises the death and destruction that he has inadvertently caused. It is quite a bleak story to start a new Doctor's era with, but it is most definitely effective