Danger Mouse: The Snowman Cometh (2015)
While most installments are only half-episode length (i.e.: 15 minutes minus commercial breaks), the season one Christmas episode runs for the full half-hour (again, exempting those meddlesome advertisements).
This isn't the Snowman's first appearance in the reboot - the character shows up periodically to be soundly humiliated by Danger Mouse - but it's the first in which he's the main villain or in which he poses any kind of meaningful threat.
This is remarked on in-world, in fact - Danger Mouse is amused when he faces the Snowman in the opening on Christmas Eve. Their fight is entirely one-sided, to the point that The Snowman doesn't even manage to scare onlookers.
With that established, Danger Mouse and his partner, Penfold, head back to their base, where they're informed some unknown villain has threatened Santa Claus. The team's scientist, Professor Squawkencluck (easily one of the series's best characters) dismisses the idea that Santa exists, at all, in a fairly rote science vs. superstition rant. But the rest of them believe, so they head to the North Pole to lend a hand.
I should probably mention that, in keeping with the show's commitment to an all-animal world, Santa's a polar bear, and his elves are penguins. This doesn't really impact the story, though, and you'll be better off if you don't think too hard about scale.
As the episode's title implies, the villain at the heart of the threat winds up being the Snowman again, but this time he gets the drop on the heroes. Before anyone can stop him, he grabs Santa's magic hat, which makes him nearly unstoppable. He takes the sleigh as well and sets out to destroy Christmas.
Danger Mouse, armed with a heat gun courtesy of Professor Squawkencluck, pursues, aided by Santa's elves (all but one of which are defeated a bit too quickly by the Snowman's newly-formed army of snowmen). Ultimately, Danger Mouse isn't enough to stop this foe... fortunately, he gets a last-minute save. After spending most of the episode in a state of depression, Santa is accosted by Professor Squawkencluck, who talks some sense into him and gives him a high-powered sleigh/spaceship.
Together, Santa and Danger Mouse manage to defeat the Snowman, recover the magic hat, and - by all appearances - outright kill the miscreant (though I'd be shocked if he doesn't reappear in a later episode).
As I said at the start, this is a solidly entertaining series, and even without the holiday elements, I think this would have been my favorite up to this point. The jokes in this one were a little edgier and a little more clever than usual, and they made good use of their expanded run time, drawing out the story without ever feeling overstuffed.
I can't say this is something you absolutely need to see, but it's fun enough you certainly shouldn't avoid it. And if you've got a special affinity for anthropomorphic animals, British spy comedies, or this particular style of animation, it's definitely worth checking out.