Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Alex Kingston (River Song)
Matt Lucas – Nardole
Greg Davies – King Hydroflax
Rowan Polonski – Flemming
Robert Curtis – Scratch
Chris Lew Kum Hoi – Alphonse
Phillip Rhys – Ramone
Anthony Cozens – Concierge
Nicolle Smartt – Receptionist
Liam Cook – King Hydroflax's body
Nonso Anozie – Voice of Hydroflax
Directed by Douglas Mackinnon
Written by Steven Moffat
Script editor David Davis
Produced by Nikki Wilson
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Length 60 minutes
Originally broadcast 25 December 2015
On Christmas Day 5343 on the human colony of Mendorax Dellora, the Doctor is mistaken by a servant, Nardole, for a surgeon hired by River Song to attend to her dying husband, King Hydroflax. River, who has not yet met the Doctor's present incarnation, fails to recognise him and increasingly frustrates him with her flirtations with Hydroflax. River takes the Doctor aside to discuss the operation he is supposed to be performing; the Halassi Androvar, the most valuable diamond in the universe, has become lodged in Hydroflax’s brain during a raid on the Halassi vaults and is slowly killing him. It transpires that River does not want the "surgeon" to heal Hydroflax, but to remove his entire head (killing him) so she can take the diamond.
They are interrupted by Hydroflax, who has listened to their conversation. Being a cyborg, he detaches his head from his mechanical body for safety and orders it to kill them. While River defends herself, the Doctor grabs Hydroflax's head and threatens to put him in the garbage disposal, creating a stalemate and allowing Ramone, River's actual husband, to teleport her, the Doctor and Hydroflax’s head outside of the ship. Believing Nardole to have information about River, Hydroflax’s body (which operates independently) decapitates him to use his head as its own.
Although the Doctor is convinced that River knows who he is, she denies it. She reveals she purposely crashed Hydroflax’s ship in their location knowing the Doctor would be in the area with his TARDIS. Being unfamiliar with his new set of regenerations, she has an incomplete set of his faces and Ramone has only been able to find the TARDIS, not its owner. River decides they will just have to borrow it instead, which she has apparently done before without the Doctor's knowledge. However, the TARDIS's safeguards prevent it from taking off when it detects that Hydroflax’s head and body, although separated, are still linked to one another. That makes the head and body both inside and outside the TARDIS.
Hearing Nardole calling for help, Ramone is tricked and uploaded into Hydroflax’s body as well. A homing beacon inside Hydroflax’s head allows his body to locate the TARDIS. Using Ramone, it forces its way inside. With Hydroflax’s head and body now both inside the TARDIS, it is able to travel to the starship Harmony and Redemption, where River requests that the maître d’, Flemming, deadlock seal the baggage hold in order to prevent Hydroflax’s body from pursuing them further.
Rather than return the diamond to the Halassi, River reveals that she intends to sell the diamond to a buyer named Scratch, who has secretly filled the meeting point with members of his own species. After receiving payment, River and the Doctor discover that Scratch and his compatriots worship King Hydroflax and are after the diamond in his honour. Despite attempts to hide the bagged head containing the diamond, they are stopped from escaping by Flemming, who was tricked into entering the baggage area by an under-duress Ramone. Flemming has promised Hydroflax’s body an even greater head - the Doctor's - and believes that River Song will be the perfect bait to get it.
Sensing that Hydroflax's head is beyond recovery and will die imminently, Hydroflax's body destroys it, leaving only the diamond. Flemming interrogates River for the whereabouts of the Doctor, but she explains that although she loves the Doctor, they are mistaken in thinking that he loves her back. She insists that the Doctor is incapable of falling in love, that he is beyond such simple emotions, and that he doesn't care enough about her to place himself in danger whenever she's in trouble. Then, upon seeing the steady gaze and mild smirk on the Doctor's face, River realises he has been with her all along. The Doctor confirms this by whispering "Hello sweetie".
Being a time traveller, River knows the ship is about to be crippled by a meteor strike, which she uses as their escape plan, taking the diamond in the process. The Doctor uses Scratch’s universal bank transfer device to overload Hydroflax’s body, before heading to the ship’s bridge. As the ship is crashing, River discovers that they are heading towards the planet Darillium - home to the Singing Towers mentioned by the future River as the place where she spent her final night with the Doctor ("Forest of the Dead"). Realising that they cannot save the ship, they flee into the TARDIS, but the impact of the crash knocks River unconscious.
Having avoided taking River to Darillium for as long as possible to delay their last date together, the Doctor decides to give in to the inevitable. After travelling to the next morning, the Doctor suggests to a man searching for survivors of the crash that he build a restaurant where they are standing, with a view of the Singing Towers, and gives him the diamond to fund its construction. Travelling forward in time once again, the Doctor books a table on the balcony for Christmas Day in four years’ time. When River awakens and exits the TARDIS into the restaurant, she is told that the Doctor is waiting for her at their table. She encounters Hydroflax’s body, since salvaged and now peacefully controlled by the heads of Ramone and Nardole, working as a waiter in the restaurant.
The Doctor gives River a present: the sonic screwdriver she will use in "Forest of the Dead". As the pair admire the Singing Towers, River asks whether the stories she's encountered about her and the Doctor - that this will be their final night together - are true. The Doctor only responds by saying, "Spoilers". He insists that there is no way to avoid the end of their times together and refuses to tell River the future. He does reveal, however, that one night on Darillium lasts for twenty-four years, giving them a long last date. As the two smile at each other, the words "And they both lived happily ever after" are displayed on-screen.