Under most circumstances, the the "special" took on greater significance, as it had the monumental task of saying goodbye to Peter Capaldi, while also welcoming incoming Doctor Jodie Whittaker, and, of course, affording long-time showrunner Steven Moffat his final turn as head writer. So it's not too surprising, then, that this year's special would be an emotional event for Who fans and its showrunner alike.

'Twice Upon a Time' delivers a fairly potent story about memory and nostalgia, one that understands the emotional attachment there is to the Doctor in all of its incarnations and how difficult it can be to let go of any one of them. Moffat introduces a number of meta-textual elements into the story, from the ache of saying goodbye to the misplaced notion of (fan and creator) ownership and even a desire to just walk away from it all and bring the whole thing to a close.

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To pull this off, Moffat positions Capaldi's Doctor and David Bradley's First Doctor in the South Pole, where, not coincidentally, they're both attempting to stave off regeneration. But as it is with Time Lords, opting against regeneration means death, something that, until fans stop watching the series is unlikely to happen. Knowing this allows Moffat to play around with his characters and show how, despite being an inherently good and heroic character that travels all of time and space righting wrongs, the Doctor is also a fascinatingly flawed individual -- one that is nonetheless capable of embracing change.

In that sense, 'Twice Upon a Time' is primarily interested in the question of why the Doctor would solider on, and, by positioning him against a much older version of himself, all while the promise of an historic regeneration into a female incarnation waits in the background, pointing out just how much the character has changed. More so than ever before, Moffat appears to dissolve the separation between character, writer, and audience, breaking down the illusion and demonstrating the degree to which the character is simply a construct.

As much as Moffat tugs at the audience's heartstrings, knowing how fond many have become of Capaldi's take on the character, he's all too aware how familiar this scenario is, hence why weighing the choice of regeneration over death, examining the power of memory, and the inadvertent hazard of nostalgia becomes the text of the story itself. That text, then, is directed as much to Moffat letting go of the position he's held for so long as it is to the audience, who, despite knowing regeneration (i.e., death of a certain interpretation of the character) is always and forever on the table, may hold too tightly on the present and have too idealized an image of the past in their heads.

'Twice Upon a Time' exists to the baton from Capaldi to Whittaker, and Moffat to incoming showrunner Chris Chibnall, and instead of playing up the ceremony of it all, focusing on the pomp and circumstance of launching a new era of Doctor Who, Moffat instead chooses to illustrate the necessity of change. More than once the episode alludes to the Doctor's age and weariness, and although it's presented as a gag – and maybe even an acknowledgment of some of Moffat's detractors – just how much society, and by extension, the expectations of its popular culture, has changed since Doctor Who was first introduced. To bring this idea home, Bradley's First Doctor is essentially a wellspring of un-P.C. lines of dialogue. Though he's still the same Doctor fans love, he also serves to demonstrate the obligation of popular culture to change and progress, lest it and its characters be stuck with antiquated notions about race, gender, and sexuality.

Rather than waive it off as behavior and opinions inextricably ingrained in an older generation, Moffat makes a point of having the present-day Doctor cringe at the person he used to be, underlining just how significant the choice of regeneration is. And, as the special makes clear time and time again, the significance is greater because of what's yet to come, as opposed to what already has been. In essence, Moffat doubles down on the idea of how change can be difficult to accept, especially when it comes to something you enjoy and are comforted by, like a piece of pop culture, whether that be Doctor Who or, as we've seen recently, Star Wars, but as much as fans may want to keep a franchise and its characters preserved in amber, they need to change and move forward to stay relevant.

As much as Moffat pulls back the curtain on Doctor Who and reveals the character as the product that it is, there's still an effective emotional undercurrent in his final outing that underscores Capaldi's time as the character. Bringing Bill Potts back as a part of Testimony, a benevolent effort to collect the memories of the dying, in a sense allowing everyone to live forever, is especially successful. The same is true of the quick, green-screen return of Jenna Coleman's Clara, which also conveniently highlights the notion that these characters are simply constructs – though acknowledging that doesn't lessen the emotional response they so often provoke.

In all, 'Twice Upon a Time' made for an effective consideration of the necessity of change from the person handing the reins over to his replacement. Moffat closes out his run on the series with the reminder that though one era has come to an end, the idea of Doctor Who will continue on for the foreseeable future, but that change is often necessary and for the better.

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Doctor Who continues in 2018 on BBC and BBC America.