'The Snowmen' struck a perfect balance between story continuity (the Doctor in mourning for Amy & Rory... return of soufflé Dalek girl...) and the whimsical (TARDIS on a cloud... Dickensian setting...)
What would make a good Christmas storyline?
Clara gives the Doctor a present and invites him to spend the day with her and Danny. But Twelve is being the proverbial 'Scrooge' - "Oh Claara, Claara, Claara. Christmas is as much superstitious hum-bug as the pagan Saturnalia festival it replaced! Messiahs and virgin births? Pah!
Clara, being agnostic, challenges the Doctor to find out the truth for himself.
They travel back to 1st Century Roman occupied Palestine during the months of the population census. There they meet 3 members of an ancient Alien race known as 'The Wise'.
They inform the Doctor that a great event is about to take place on Earth: The Creator is about to take physical form as a human. This event has taken place on other Planets, among other alien races, always at crucial/pivotal times in their history.
The Doctor is skeptical but intrigued.
However, a shadowy cult has also come to Earth to prevent this from happening.
Their leader has set himself up as the Hebrew King and has ordered the slaughter of the newborn.
Because the 'Wise' abide by strict laws not to reveal themselves to still primitive worlds, they are restricted in what they can do. However the Doctor and Clara, being familiar with the 'legends', use the TARDIS as a beacon of light in the sky to guide them to Bethlehem. Thus, the TARDIS becomes the guiding star.
In Bethlehem, the Doctor, Clara and the Wise discover an exhausted young couple taking refuge in a rundown, smelly cattle shed. "Not exactly the cosy Christmas card scenes is it Clara!" quips the Doctor. However, the Wise are seemingly overcome with emotion at this squalid scene and begin worshiping the newborn child of the young couple. The Doctor surreptitiously scans the child with his sonic screwdriver and his face turns white!
Seeing his expression of fearful awe, Clara demands an explanation.
Oh, his DNA is 100% human. But there's something else... On Gallifrey, they called it the Origin Enigma: When Timelords traveled to back to the beginning of the Universe, they detected a mysterious force which triggered the Big Bang. That force... It's in this child!
After their adventure, the Doctor agrees to spend Christmas with her and Danny. Looking at the Christmas cards with scenes of wise men, guiding stars and mangers, now infused with new meaning, the Doctor smiles wryly to himself.
And when Carolers come to their door singing Silent Night, the Doctor joins in.