Warning: spoilers for the Doctor Who Christmas special "The Return of Doctor Mysterio" are ahead!
Ever since the Tenth Doctor met her in 2008's "Silence in the Library," River Song has been an important player on Doctor Who, even though, as The Doctor once admitted, you need a flow chart to keep up with their relationship. However, last year's Christmas special, "The Husbands of River Song," effectively wrapped up her storyline with the adventuring Time Lord, as they finally made it to the Singing Towers of Darillium, the last place River will see him before her death in The Library. One year later, tonight's Christmas special, "The Return of Doctor Mysterio" closed the entire book by providing an epilogue to their time together, with the Twelfth Doctor finally ready to move on to the next chapter of his life.
While the main action in "The Return of Doctor Mysterio" revolved around The Doctor's adventure with Grant Gordon, a.k.a. The Ghost, and their fight against sentient, body-snatching alien brains, the Twelfth Doctor was also using that time to still come to terms with River's death. As those of you reading this already know, Doctor Who Season 10 didn't air in 2016, so aside from the Twelfth Doctor's appearance on the spinoff Class, we haven't seen him since "The Husbands of River Song," exactly a year ago. Well, it turns out it's been a long time since The Doctor has been getting into shenanigans in the TARDIS in this universe, too, as it hasn't been that long since parted ways with River. Unfortunately for him, her death (or coming death, depending on how you look at it) is still weighing heavily on his mind.
Our first indication that something is amiss with the eponymous protagonist when Grant mentioned how's he known Lucy Fletcher, the woman he loves, since elementary school, i.e. 24 years. i.e. the exact amount of time The Doctor and River Song spent together on Darillium. Later in the show, Nardole, River Song's associate from last year and The Doctor's newest traveling companion, refutes The Doctor's claim that he cut him out of the Hydroflax robot because of his researching skills, citing that the real reason he did so was because he "didn't want to be lonely," to which The Doctor doesn't reply.
Finally, once the evil brains have been defeated and New York has been saved, that's when Grant and Lucy (who are now romantically together) realize that something is troubling The Doctor. While he isn't willing to go into specifics with them, he does say this:
Once The Doctor has left the room, that's when Nardole summarizes to Grant and Lucy how The Doctor is still getting over River Song's death. When they ask if he'll be okay, Nardole responds:
If the Doctor were still in his Eleventh incarnation, we could hold out hope that he'll run into River again at some point given the timey-wimey nature of their relationship. However, it's important not to forget that before "The Husbands of River Song," River had never seen the Twelfth Doctor. When she was looking for The Doctor in that special, she was still expecting him to be using one of his previous 12 faces. That means her first time with the Twelfth Doctor was also the last time, and the next time she runs into him, it will be his younger Tenth self who's never seen her before. The Twelfth Doctor knows this, which is why he's silently coping with this loss, but by the end of the special, it looks like he's ready to continue his life as a madman in a big, blue, bigger-on-the-inside box.
Since the Eleventh Doctor also bid farewell to River digital "spirit" in 2013's "The Name of the Doctor," now the River Song saga is essentially finished. It's possible Alex Kingston may reprise the character in more Big Finish audio dramas, but as far as the TV show goes, the chances of her returning are extremely slim, which is sad, but also okay. Doctor Who has only survived this long because of its willingness to change, and while many fans will fondly look back on the River Song times, the show is now moving head to new destinations and horizons.
Doctor Who Season 10 is scheduled to air sometime in spring 2017 on BBC America, and will see The Doctor traveling with new companion Bill (played by Pearl Mackie). Matt Lucas will also appear as Nardole on a regular basis. In the meantime, let us know what you thought of "The Return of Doctor Mysterio" in the comments below.