Warning: plot spoilers for Doctor Who Christmas Special Joy to the World.Doctor Who is back with high-energy, heartfelt festive special Joy to the World. Being a Steven Moffat joint, its full of wild and inventive concepts, from time travelling hotel rooms to psychic briefcases, via bootstrap paradoxes and sentient stars.Oh, and the Doctor spends a year serving breakfasts and fixing microwaves.But dont worry if you were so deep in a food coma that you missed a key detail here and there, because were here to unpack some of those concepts and answer any questions you might have about the episode.Starting withHow Does The Time Hotel Work?A hotel, but instead of rooms, time portals package deals for all of historys biggest hits.Its kind of amazing Doctor Who hasnt used the idea of a hotel based on time travel before in fact, we wouldnt be surprised if Bad Wolf Ltd announces a spin-off anthology show built around the concept (and if they havent thought of it yet, and are inspired by this piece, wed like our cut of the profits please).Guests checking into the Time Hotel can use its various rooms to visit different periods in history, from ancient Mesopotamia to (somewhat morbidly) their favourite historical assassinations. Sensibly, Moffat doesnt get too complicated and timey-wimey with the idea the timelines in the various rooms all seem to be running discretely in parallel, so you cant check into a room until the contemporaneous guests vacate. The rest of the hotel like the front desk and the bar follows a single linear timeline, with a clearly defined present, to avoid paradoxes.Well, apart from the kitchens, which are 30 minutes in the future, so they can deliver your food milliseconds after you order it. Which is just good hospitality, when you think about it.How Did The Doctor Get The Code For The Briefcase?The Doctor of the past is given the code for the briefcase by his future self, who knows the code because he remembers it being given to him by his future self. This is known as the Bootstrap Paradox, an event that effectively has no origin point, because the presence of time travel gets around the need for cause and effect.As the Doctor puts it, basically, the code came from nowhere but then so did the universe, and no-one complains about that.Join our mailing listGet the best of Den of Geek delivered right to your inbox!Steven Moffat has used this idea several times before, notably with the Doctors escape from the Pandorica in series fives The Big Bang. Hes rarely been quite as brazen with it as he is in Joy to the World, though, cheerfully lampshading the impossibility of the resolution through the Doctors brief exchange with Joy.How Did Trev Come Back?With technobabble, thats how!Alas, poor Trev (Joel Fry). In one of the episodes more brutal touches, the Doctors hapless would-be partner dies suddenly, painfully and alone, his promise not to let his new friend down unfulfilled.But, this being Christmas, he gets a second chance courtesy of the handy psychic graft that Trev mentions at the beginning of the episode, a communicator implant so sensitive that it automatically calls his mother if he forgets to flush the toilet.Luckily for poor Trev, having been telepathically connected to the briefcase containing Villengards baby star, his psychic graft interfaces with the case, duplicating his consciousness and uploading a virtual version into the cases communication software much like the hologram version of Splices soldier father in Villengards previous appearance, last series Boom.Now functionally immortal, A.I Trev is able to spend millions of years inside the briefcase, patiently working out a way to jump across to the Doctors sonic screwdriver (or, as Nicola Coughlans Joy puts it, his zappy thing) and fulfil the promise he made millennia ago in the future a year or so back.Everybody lives! Sort of.Read more Why Did The Doctor Spend A Year At The Sandringham?Unfortunately, if you park your TARDIS in the Time Hotel, then use one of the Time Hotels rooms to visit a different hotel one year in the past, through a door that your future self promptly seals shut, youre obliged to go back to the future the long way, i.e, by living out 365 consecutive days in full linear order.Though luckily that allows for a delightful montage in which you fix microwaves by making them bigger on the inside, fix car sat navs so that they take the driver where they need to go rather than where they want to go, and learn the importance of chairs.And friendship, of course.What Did Joy Do With The Star Seed?Similar to Trevs unexpected second life, Joys climactic union with the star seed requires a certain amount of handwaving.Essentially, the psychic connection shared between Joy and all the other personalities killed by the briefcases security system allows them to override Villengards technology, join together and become one with the star seed. As Joy says, The star seed is in me now Villengard are nothing. Were far beyond them.The star is not just a star now its a composite entity, made up of the star seed and all the psychic echoes of the people within it. And together, theyre able to fly far away from Earth, where they can do no harm, and become a shining beacon of comfort in the night sky.And apparently absorb the essences of anybody else they want to, like Joys mother.On the face of it, this resolution is rooted more in poetry than in science-fiction. But, as famously pointed out by Anne Hathaway in Christopher Nolans Interstellar, love is the one thing were capable of perceiving that transcends dimensions of time and space.And shes a scientist.Hold On, Was That Bethlehem At The End?Indeed it was. Steven Moffat is no stranger to scribbling mischievously in the margins of history, but this might be his most audacious and potentially controversial scribble yet.Yes, the star that guided the three wise men and a motley entourage of shepherds, angels, donkeys and kids dressed like sheep, if the average nativity is to believed was actually an experiment by an evil weapons corporation from the future.Its cheeky, for sure. But its also, in its way, perfectly Christmassy. Something with corrupt and evil origins that, through the power of love, becomes a symbol of hope, shining through the centuries.Joy to the world, indeed.Doctor Who Christmas Special Joy to the World is available to stream on BBC iPlayer and Disney+.