The Doctor has had many epic moments in the 50 years that Doctor Who has been on the air. But this moment featuring Patrick Troughton isn't just one of the most epic, but possibly the most important of all. This is the moment where the Doctor announces his mission to battle evil for the first time.

The above clip comes from "The Moonbase," a 1967 story that's just out on DVD, and it's a significant story for a few other reasons as well. For one, it features the second appearance of the Cybermen — and they're way more menacing and awesome than in their first appearance in 1966's "The Tenth Planet." For another, it features Troughton starting to find a balance between humor and intense seriousness, after a few stories in the role of the Doctor, the mysterious time traveler.

But the above scene seems not just iconic, but significant — because not only is the Doctor saying that "the most terrible things" in the universe "must be fought," but he's also stepping up and saying he's the one to do it. Hobson, the commander of the Moonbase, wants the Doctor and his companions to get off the Moon, but the Doctor refuses to, because there is evil here.

Contrast that with most of the William Hartnell era, in which the Doctor is usually unable to get back to the TARDIS because someone has stolen it, or there's a force field, or it's broken down, or one of his companions is a prisoner somewhere. The Doctor gets dragged into an adventure because he's stranded somewhere.

It's only in the last year or so of Hartnell stories that the Doctor doesn't need to be forcibly separated from his time machine — there's nothing keeping him from leaving in "The War Machines," except that Sir Charles asked for his help. But here, the authority figure in charge is telling the Doctor to leave, and he's refusing. It feels like a bit of a sea change.

But also "The Moonbase" feels like a story in which all the classic Who elements start to gel in lots of ways. It's the first "base under siege" story, and the first time any monsters besides the Daleks really feel menacing — although the very next story, "The Macra Terror," does a lot to keep raising the bar, judging from the clips that survive. But it also shows how much more of an instigator Troughton's Doctor is becoming, right out of the gate, as he pokes around looking for trouble to get into.

The good news is, the new DVD of "The Moonbase" is the first opportunity most of us have to appreciate this story in its full majesty. Episodes one and three of this story are missing from the woefully incomplete BBC archives. Somewhere in a cardboard box, I have a VHS tape of a fan-made "reconstruction" of those two episodes, and you can also get the audio with linking narration. But now, the two missing episodes have been professionally animated, to the original audio that survives, and it's way more watchable. (Similar to the DVD releases of "The Invasion," "The Ice Warriors," "Tenth Planet" and a few other stories.)

That said, the animation feels a bit slow in parts, even by the standards of 1960s Who — and I think part of it is just that you can't see the expressions playing against Patrick Troughton's face, and some of the other actors doing physical acting. The animators really try to make the faces look expressive and mobile, but it's not the same thing. The Cybermen look pretty awesome in the animated versions, however.

And like I said, "Moonbase" is brilliant — it's as good as the other Troughton Cybermen adventure from the same era, "Tomb of the Cybermen." The Cybermen, who were a bit dense and feeble in "Tenth Planet," are suddenly a lot more cunning and relentless in their second outing. (Similar to how much better the Daleks are in "Dalek Invasion of Earth" than "Dead Planet," their first story.) With episode three now in place, you get to see the Cybermen take over the Moonbase before being ejected, and that leads into their iconic march across the Moon's surface.

The DVD extras are relatively sparse, by the standards of recent releases — which is actually not a bad thing, since some of the hour-long featurettes on the classic Who discs can get a bit hard to sit through. There's a "making of" documentary, plus production subtitles.

And you come away with the sense that this was an unusually challenging story to pull together — the sets, including a Moonbase and the Moon's surface, are incredibly ambitious. And the sets weren't ready in time for the first episode's filming — they had to film the Moonbase control room scenes with the paint still drying and some magazine cutouts in place of actual controls. And the fourth episode was unexpectedly moved to the dreaded Lime Grove Studio D, a tiny cramped space where the earliest Doctor Who episodes were filmed — requiring a lot more ingenuity to pack that much action into a teeny space.

And you learn that director Morris Barry made a huge point of toning down Patrick Troughton's humor and trying to bring out the serious side of the Second Doctor, resulting in moments like the one at the top of this post.

We've come a long way from the time when there were only a handful of Troughton stories still available in any format — more episodes have been recovered, and other episodes have been recreated using animation. With the rediscovery of "The Web of Fear" and "The Enemy of the World," plus next month's half-animated release of "The Underwater Menace," the Troughton oeuvre is starting to look downright respectable.

And unlike a lot of times when a lost classic resurfaces, for the most part the Troughton era holds up just as well as you always hoped it would. He's the scrappy champion of good and justice who leaves chaos in his wake and confounds the monsters of the world. Moments like the one up top, where he suddenly reveals the steely seriousness under his comical exterior, are part of what make it all worthwhile.