There is much to enjoy about this serial and, in terms of Doctor Who history, there is much to make note of. The plot sees the Doctor arriving on Earth in the middle of a sinister plot by International Electromatics and assists the military taskforce UNIT (as led by Brigadier Lethbridge-Stewart) in identifying and countering the threat. It should be said early on here that I didn't watch the animated versions of the missing episodes – only those originals that remain. As a result of this I think I was made out of it a little bit as can often happen when chunks of the serial is missing. The second thing to say is that the audio and video quality of some of these episodes was really very poor, so again another obstacle to enjoying it.
I say this as a caveat because I thought The Invasion was a mixed bag. The focus is very much on the human side of things, with the introduction of UNIT and the coordination from IE allowing for plenty of dialogue scenes. Many of these are pretty good but too much of the plot is based on convenience and coincidence and this undermines it to a certain extent. Beyond this the dialogue driven scenes don't have quite the impact they should and this is something which is highlighted by some of the stronger ones but also the couple of very good cliffhangers involving the Cybermen. When they finally show up, their appearance is dramatic and they visually leave an impression in the breakout of the pod, the dark sewers and walking through the streets of London. That said it doesn't quite have the impact of the Dalek invasion serial, but it is still good and the danger and urgency they bring is appreciated.
The ideas behind UNIT are fine and I hope they will be well used moving forward – obvious I know them from the Who era I have already seen, so it was interesting to see them starting out here. The performances are a bit mixed. The UNIT people generally add seriousness to the proceedings (particularly Stewart) but the rest are hit and miss. Some of the character give the thing a bit too much of a "swinging 60s" feel which takes away from the drama. Troughton is solid for the most part but the lingering image I have of him here is him running from the Cybermen grabbing his rear end like it is being burnt – which is misplaced comedy in my opinion – add to this him modeling on the floor seconds later, and to think people complain about Matt Smith (albeit such things are his default). Padbury and Hines are both good here and generally work well. The Cybermen are too remote a presence for too much of the serial and, when they come, they maybe don't have the menace they could have done.
The Invasion does have much to like but it never really feels like it gets the most from all its parts. It does have a nice structure and some good cliffhangers but it doesn't sustain the sense of danger and urgency that it needed to be really strong. A case of "almost" for me in this case.