Invasion of Time wants to be an epic story. And it sets out all the elements to be just that. Unfortunately that ambition is repeatedly undercut by budget and other factors.
I've been fairly critical of the 6 part story format for classic Doctor Who, because more often than not it results in a lot of padding. In this case however, there's no shortage of plot to fill all six episodes. Unfortunately it seems they had money for no more than 4.
As a result we get wildly variable production values. The scenes that feature Gallifrey take their cues from The Deadly Assassin and therefore look great. On the other hand, the scenes of the Doctor being chased around the TARDIS interior look like they were shot in an empty hospital. And they look that way for a very good reason...
In similar fashion the acting quality is all over the map. On the one hand we have John Arnatt and Milton John as Borusa and Kelnar who are excellent and wonderful foil's for Tom Baker's Doctor. On the other hand... well just about everyone else.
And Baker himself embodies this variance. Sometimes he is giving a subtle and powerful performance, and then he completely undercuts it my hamming up directly to the camera. This is the story with the infamous "not even my sonic screwdriver can get me out of this" line. In many ways this is the point where Baker's Doctor starts to become a caricature of himself.
Somewhat unusually the story actually features two sets of villainous aliens. The first ones, the Vardans are interesting in concept, but once again the budget lets us down and they are visually... people covered in aluminum foil. The second are the Sontarans. And here you can see the beginning of their descent into comedic characters. They are at least still dangerous here, but they're also incredibly stupid and incompetent. I know I bang on about this, but they are a wasted opportunity.
I guess the other thing I need to mention is Leela's departure. It is bizarrely tacked onto the end of this story and nothing we've seen feels like we've actually lead up to it. It doesn't even seem like behavior that's remotely in character for Leela.
All in all The Invasion of Time is a frustrating story to watch. There is so much potential here and some moments of pure magic. And then it promptly stuffs it up.