Doctor Who fans get excited about Christmas for entirely different reasons than those who don't watch the series: each year, a new special episode airs on Christmas Day, usually featuring a fun and joyful storyline that highlights the nature of the holidays.

This year is no exception, although fan expectations are higher than ever, thanks to a stellar season of the series with Peter Capaldi on board as the 12th Doctor. Many fans claim this was the best season ever, and expect great things from this year's Christmas special.

Fortunately, the BBC has released some details and photos that give fans an idea of what they will see, so here's a list of everything we know so far about this year's Christmas episode.

River Song

Early on, the BBC announced that River Song (Alex Kingston) would return to the series for the Christmas episode. However, as the BBC released more information, fans soon realized that things are different between River and the Doctor now, particularly as showrunner Steven Moffat confirmed that this time, River doesn't immediately recognize him.

"The Doctor is an all-new man and has been for a while," said Moffat to the BBC. "It may have slipped his mind that out there, in a very tangled and complicated way, is his wife that has never seen this face before and doesn't even know about this incarnation."

However, the episode is titled "The Husbands of River Song," meaning that she's now got more than one husband (she married the Doctor back during his 11th incarnation).

For those following the series, River and the Doctor operate in a wibbly wobbly world of timey wimey and their chronology is often completely contradictory. Since her first appearance in the series in "Silence in the Library" with the 10th Doctor (David Tennant), fans have wondered which Doctor was her first (and the one she first fell in love with), and hope that Peter Capaldi's version of the Doctor is it.

The Villain: King Hydroflax

Of course, a Doctor Who episode, particularly a fun Christmas one, needs a villain, and this time around it's King Hydroflax (Greg Davies), a huge cyborg that's out to ruin the Doctor and River Song's fun. However, through most of the episode, the only thing we see of King Hydroflax is his detached head: the rest of his body has gone rogue.

"One of my favourite scenes is this speech where I tell the Doctor that I'm going to 'BURN THE WORLD' in very dramatic, Armageddon-like terms," said Davies to Radio Times. "And then Douglas [Mackinnon], the director, pointed out that I'm just a head when I'm threatening this, which is even funnier. Then he put it that I'm just face down in a grill in the Tardis, so it's the most pathetic threat of all time from a disembodied head, with his head in a grate, screaming, 'I'm going to destroy you all!' "

However, there's one more twist to the story. That other husband of River Song mentioned in the episode's title? It's King Hydroflax.

Matt Lucas

Unfortunately, all we know about funny man Matt Lucas in the Christmas episode is that his name is Nardole and that he's dressed for cold weather in at least one scene with River Song. The BBC has kept his identity under wraps, but it's likely that he'll be hilarious in whatever role he turns up in. One thing given away in trailers, though: he's looking for a surgeon of some kind.

The episode description asks "will Nardole survive?" which means that his life is, apparently, in some kind of danger. Of course, it's Christmas, so it's likely the Doctor saves him from whatever threat he faces.

The Christmas Episode Is Mostly Cheerful

Having recently lost a companion, the past few episodes of Doctor Who have required fans to stock up on the tissues. Not only did fans see Clara Oswald (Jenna-Louise Coleman) die, but then they saw her brought back to life only to have the Doctor completely forget her before she goes off on her own adventures with Maisie Williams' Me. Moffat loves to tug on those heart strings, after all.

However, with the Christmas special, Moffat has promised fans that it won't be dark and depressing like the rest of the season.

"It kind of is different, it has to be Doctor Who," said Moffat to The Mirror. "But particularly after the end of the last series where it got quite dark, we had to go back to the light-hearted action and adventure, with a kick of emotion."

The Doctor Who Christmas episode airs Dec. 25 on BBC America.