The episode was on point.
Riversong (Alex Kingston) has always been one of the most interesting story lines of Doctor Who. Spanning across three (now) Doctors, she knows him better than anyone. She is his wife. However, he may not be her only husband.
However Riversong is unaware who The Doctor is, having never seen the Capaldi-Special, a similar emotional tug at the heartstrings as at Silence in the Library back in the Tennant days. It was in The Library of course that we first met Riversong and the adventure began. More of that later.
As the episode is titled The Husband's of Riversong, she has two husbands in this episode, apart from The Doctor, and this adds a different dynamic to her relationship with him. Capaldi gets his fair share of jealous-husband-syndrome before declaring his love for Riversong - without actually voicing it of course - in the final moments of the episode.
Cameo two, and one half of Riversong's new husband duo, King Hydroflax (Greg Davies), is a human head on a large red robot. A human head with a very expensive diamond embedded in his brain, thus laying out the base story line for the episode.
When Riversong visited The Library and met The Doctor (Tennant), we had no idea who she would become, and neither did he. Watching back now, that two-parter means so much more and this episode mimics those very first moments. She spoke of visiting the Singing Towers of Darillium, and we knew that was where their story would always end. So when their intergalactic cruise ship begins hurtling toward Darillium, Whovians everywhere held on to their fezes.
Riversong reminds The Doctor that he often promises to take her the the Singing Towers yet always cancels, reminiscent of The Doctor's inability to say goodbye.
And then he hands her a small gift box. A gift box containing a Sonic Screwdriver. A Sonic Screwdriver with dampers.
So his time with Clara has been a lesson learned, and it seems our Doctor, who previously was so against letting go, realises that he can't save everyone.
This scene was a long awaited, fun addition to Doctor Who that we will all be quoting for seasons to come.
Perhaps this will be the last we see of her ...
Who am I kidding. They are time travelers!
... Right? ...
What a year. What a season. What a Christmas Special. New species, robot-bodied-heads, Riversong and reminders of Doctor Who past.
The only thing missing was Missy.