The Eighth Doctor has promised Lucie a nice Christmas dinner. He also invites [[Recap/BigFinishDoctorWhoNEDAS4SRAnEarthlyChild Susan and Alex]]. It's been six months in their relative time, and Earth is recovering and finally reaching out to interstellar contact again (with a bit of anonymous help from the Doctor).
They all spend a long time catching up, decorating the TARDIS, enjoying the food, getting to know each other better, and ignoring the distant anguished wails of what sounds suspiciously like the Doctor himself. When a giant fish that swims through air arrives to spoil their party, they simply lock it in and really try their best to enjoy a very quiet Christmas. Lucie gets to know Alex, who's coming to terms with being part alien and is looking forward to a career in architecture. It turns out that Alex is only 7% Gallifreyan, not telepathic and not regenerative. The Doctor's a bit disappointed to hear that.
The Doctor shows Susan her old room, which he preserved, along with all other rooms of all companions that came after her. However, Lucie figures out just in time that if the giant fish is of the same species as one that Susan used to own once, it might be the same fish. Meaning it's in Susan's room, and will be let out once the Doctor opens Susan's door. He does so (fixed point in time, since the later version of the fish is already with them) and realises that his echoing screams will result from him becoming fish bait soon and clinging to a time-hopping, pandimension resurrectional air fish for dear life.
While that's going on, Susan sees that the Doctor's already created a room for Alex, before even asking him if he wants to become a companion. The Doctor opens a telepathic link to her while clinging to the fish, hoping to talk to Alex ("I can't pass you over! ... because ''he's not telepathic'', Grandfather!") and eventually gives instructions that allow the others to save him. The fish, who never intended any harm, is simply let off the hook and thrown back outside.
The Doctor enjoyed his fishing trip with his great-grandson a lot, but before he can ask him to become a companion, Alex and Lucie announce that they'll be travelling 22nd century Europe together. They all agree to meet up for Christmas dinner in a year, and the Doctor glumly keeps the fact that he got a cool new TARDIS key for Lucie (with Christmas music function and everything) a secret. Alone again, he wishes himself, and anyone who may be listening, a happy Christmas.
[[Recap/BigFinishDoctorWhoSRTheFourDoctors "The Four Doctors"]] takes place directly after this.
* AllThereInTheManual: When Radio 4 Extra broadcast the season, they didn't include "An Earthly Child", so Alex pretty much comes out of nowhere here. Thanks to the Creator/BigFinish website being a ''freaking labyrinth'', "An Earthly Child" is still damned hard to find if you don't know you're supposed to be searching for it in the first place.
* CompoundInterestTimeTravelGambit: The Doctor's undrawn UNIT salary has accumulated a lot of interest over the years. His debit card helps the Christmas Shopping.
* ContinuityNod: Lots to the time where Susan was on the show, a name-check for all companions up to Victoria, and also nods to "Death In Blackpool" and "[[Recap/DoctorWhoS1E1AnUnearthlyChild An Unearthly Child]]".
** Lucie asks Alex if he's half alien, or a quarter alien, or whatever -- a nod to the whole [[Recap/DoctorWhoTVMTheTVMovie "half-human" fiasco]].
** The Eighth Doctor still [[Recap/DoctorWhoTVMTheTVMovie gets overly excited about footwear]].
** The Doctor tries to neutralise the fish using a Blinovitch Limitation Effect. It fails.
** The fish was taken from a bridge market on Quinnis, a place in the "Fourth Universe" mentioned in [[Recap/DoctorWhoS1E3TheEdgeOfDestruction The Edge of Destruction]], before appearing in the Companion Chronicle "Quinnis", released the same month.
* ForeShadowing: For the season finale. Alex is not regenerative.
* HalfHumanHybrid: Slightly averted, it turns out Alex is only 7% Gallifreyan.
* InternalHomage: Christmas, a scared giant air fish, and a TimeyWimeyBall. 2010's Doctor Who Christmas specials were [[Recap/DoctorWho2010CSAChristmasCarol eerily similar]].
* LeaningOnTheFourthWall: The Doctor pretty much wishes the audience a happy Christmas while talking to himself, a ShoutOut to the First Doctor doing the same during the "Feast Of Stephen Episode" of "The Chase".
* LighterAndSofter: Considerably happier than any other episode in season 4.
* MythologyGag: The Doctor talks about the time he took Leonardo da Vinci to see the birth of Jesus, in a riff on the short story "Innocents" (also written by Marc Platt)... only that featured the AU Doctor from the Unbound audios "Auld Mortality" and "A Storm of Angels", and goes a bit differently.
* NeverTheSelvesShallMeet: The Doctor tries to weaponize this on the fish, hoping they'll cancel each other out. It fails.
* SnowballFight: Lucy and Alex have one.
* TimeyWimeyBall: The creature is loose and causing touble on the Tardis before it has been released chronologically.