The Time of the Doctor is a regeneration story, bringing the TARDIS tenure of the Eleventh Doctor to a close. It also brings to a conclusion the entire Moffat era up to this point, in that it draws together elements which first appeared back in The Eleventh Hour and which have run throughout Series 5, 6 and 7.
In some ways you could argue that the story really begins with Rose, as it includes a postscript to the Last Great Time War.
That is potentially going to be reignited if the Time Lords return to the Universe - last seen being saved by being deposited in a pocket time zone by the multiple Doctors in the 50th Anniversary story.
The mechanism by which the Time Lords seek to find out if it's safe to return just happens to be the crack in Space / Time which the Doctor first spotted in Moffat's first story in charge, the one in Amelia Pond's bedroom and through which people and things disappeared - the story arc for Series 5.
The thing which will let them know that is safe is a certain code - the Doctor's real name, which has been the arc for Series 7.
Along the way we meet old foes, as well as yet another of those long-term friends of the Doctor whom we have never actually met - something which Moffat has been introducing ever since gathering his Demons Run gang.
The main location for the story is Trenzalore once more, which has been referred to since The Wedding of River Song and was finally seen in The Name of the Doctor.
Then, we saw the planet devastated after the events of this story, but in an alternate timeline in which the Doctor failed.
As well as a regeneration story, this is also a Christmas Special, so the village setting is a town called Christmas, and it's permanently snowy and festive looking.
If we go back to Dorium's words in the Series 6 finale, there's reference to no living soul speaking falsely - and that's covered by the crack emitting a truth field. This is because the Time Lords want to make sure that it really is the Doctor giving his name.
The new / old friend is Tasha Lem, who is basically a surrogate River Song - old friend who is flirty with the Doctor, that kind of thing.
She is in charge of the Papal Mainframe - the militaristic Clerics having been introduced in Time of the Angels / Flesh and Stone. She provides a hefty info-dump which explains a lot of leftover bits of Series 5 and 6.
We learn that the Silents are the way they are because they are confessors - you'll happily tell them your sins because you'll forget about reliving them once you're done.
Kovarian (Series 6 arc) was head of a schismatic breakaway faction who wanted to prevent the Doctor giving his name on Trenzalore, because it was claimed that silence would fall if he did (i.e. the Time War would start up again as there are Cybermen, Daleks etc waiting to either prevent this or recommence hostilities). One of her schemes was the destruction of the TARDIS (Series 5 arc).
As well as them, we also see Sontarans and Weeping Angels (despite the Sontarans never having been involved in the War, as clearly stated in The Sontaran Stratagem / The Poison Sky). Other aliens get name-checked and we recognise some of the spaceships such as Judoon and Silurian.
These enemies are all there basically to provide cameos for Smith's finale.
Nods to the classic era include the Doctor using the Seal of Rassilon (confiscated from the Master by his third incarnation in The Five Doctors) to translate a message emanating from the crack.
The whole fixed regeneration idea introduced by Robert Holmes in The Deadly Assassin is addressed her. Pre-Timeless Child, there is a 13 lives limit, and thanks to the War Doctor and a controlled regeneration by Ten, the Eleventh is actually the Thirteenth.
It turns out at the conclusion that Time Lords can issue a new regeneration cycle (as offered to the Master in The Five Doctors), but Moffat wisely declines to let us know how many new lives the Doctor has. We assume at this point that it's another 13, which should keep the series going for another 50 years (or 39, if actors keep quitting after 3 series).
One of the entertainments which the Doctor lays on for the children of Christmas is a puppet show, and one of these is modelled on a Monoid, from The Ark. He has obviously recounted many of his adventures as the kids have made lots of drawings, and we see creatures from both era of the series.
All of the aliens get beaten until only the Daleks are left. They employ converted human drones as introduced in Asylum of the Daleks.
When he first goes aboard their spaceship, the Doctor is dressed in a cloak - a reference to the Harry Potter franchise.
Karen Gillan makes a cameo appearance just before the regeneration. Interestingly, both she and Smith are wearing wigs thanks to filming on movie projects (Gillan shaved her head for her role as Nebula in a Guardians of the Galaxy outing). We don't get to see Caitlin Blackwood as Amelia, as the actress had grown too old and no longer resembled her younger self.
Drawings in the TARDIS are of creatures from Amy specific stories, like Saturnynes.
Clara sees the TARDIS phone dangling - which we'll learn about next time.
That will be when we take a deep breath and launch into the adventures of the Twelfth Doctor and the second half of the Moffat era...