The plot makes a nice point of the problems of time travel, which is pretty surprising for Classic Who. In my dealings so far I've got the impression that it was practically ignored. Of the serials I've watched so far I can only think of City of Death that pulls something similar. Mostly time travel is a means to an end, a way of getting the Doctor to that particular story. Here, it's front and centre. Which might be one of the reasons it's so good.
It's certainly not the Ogrons, the new race introduced to do the Daleks' grunt work. A small part of me can't help but think that they exist because the producers decided that the Daleks were too cumbersome. Bi-pedal men in rubber masks are far easy to direct that pepper pots on casters. However, they're never developed. They give the humans a way of doing things behind the Daleks' back. The final confrontation with the Controller just wouldn't have worked if he had Daleks backing him up. If it was just this story I'd forget it as a one-off, but I know they're back in Planet of the Daleks.
However, between the Ogrons and clever use of indigenous population keep the Daleks at a distance, allowing their evil to hang over everything without actually compromising their scariness. It's pretty rare that the Daleks actually manage to have much menace attached to them. After The Dalek Invasion of Earth they were used too openly. Even New Who only really managed it with Dalek, so it's good to see that it can be done.
With the time travel and Daleks it also allows Jo, who being mostly Earthbound so far, to see the Doctor in a different light and stretch her companion muscles a bit. She's not used to this side of the Doctor. She's not been with the Doctor when he arrives in a totalitarian government and sets about opposing the oppressive regime. The Doctor helps the authority types like UNIT. By separating them during the time when the guerilla's were hostile Jo is easily persuaded by the Controller that he's a good guy, missing the fact that humans are repressed and that the Ogrons are doing most of the security work. It makes the companion taking the wrong side trope incredibly believable.
Again, I once promised I'd try and not moan about the budget of Classic Who, or at the most a quick mention of the poor costume design, coughicewarriorscough. However, certain aspects of Day of the Daleks does need mentioning. The end battle is a bit of a joke. The disintegrators as a whole are pretty ridiculous. Clearly a budget saving device with a gun that only makes sound instead of actually firing anything. Then you have two or three Daleks and a small group of Ogrons apparently running roughshod over a well entrenched UNIT force. We see shots fired, but not even one of the Ogrons falls, or even stumbles. A little choreography would have improved it a little. Instead it looks like a bunch of extras walking across a field.
Even with my reluctance of Pertwee, Day of the Daleks is certainly one of his better serials. It's not done much to improve my opinion of his Doctor, but he's starting to become someone I can tolerate rather than just hate his entire era, which I'd gotten very close to before. Like Seeds of Death, Day of the Daleks manages to go epic and restore my faith in Classic Who. Despite it's budget.
However, this has made me realise something about the Daleks. During The Chase I complained that upon discovering the secret of time travel they used it for revenge on the Doctor instead of conquest, which seems something so much more Dalek, even back then. Again Power or Evil might actually make sense of this, but Day just muddies it more. Here they have used it to travel in time to strike at Earth at a vulnerable spot. Exactly what I would expect them to do. So how come we don't have a galaxy taken over by them? They can go anywhere and when. Future story Destiny ignores this and concentrates on a very linear war. Resurrection has a time tunnel so at least acknowledges this ability, but this is specifically after the Movellan war and the defeated Daleks are using it to reach Davros in hope of a cure. So here we an example that their time travel is a pretty late discovery, at least near the end of the time as a major galactic power.
The Daleks talk about their ability to time travel here to the Doctor like it's a revelation. Maybe this is an early version of it. It's certainly not as advanced as the one we saw in The Chase that can travel to different planets as well, much like the TARDIS, this is very much like the time tunnel in Resurrection. One spot to the other, like the Delorean from Back to the Future for a non-Who reference.
In fact I can't help but think that the moniker 'time tunnel' comes from a twelve year misremember of this very serial. The tunnel is used as a stable point, as it exists in the 70s and 200 years later. The tunnel and time travel become synonymous towards the end.
The Chase could even be from further in the future than all of this. The one stumbling block might be that the Daleks think Vicki is Susan, which is a major turning point in that serial. But then the Daleks never encounter Vicki again, and every Dalek on that mission is wiped out. There's nothing to say that this is from much later in the Daleks timeline, going back and trying to wipe out their greatest enemy long before he was a huge problem. The reverse Genesis of the Daleks. Even perhaps a move in the Time War, just like Genesis and Remembrance have been retconned into.