Say Goodbye to the Tenth Doctor Starting with Tonight's "Doctor Who: The End of Time (Part One)" on BBC America

The inevitable and the inescapable have arrived.

Tonight's Doctor Who Christmas special, Doctor Who: The End of Time (which airs at 9 pm ET/PT), signals the end of the Tenth Doctor's run on the sci-fi series... and that of series lead David Tennant, who will leave the series following next week's concluding installment.

I've remained (and intend to remain) spoiler-free leading up to tonight's broadcast of Doctor Who: The End of Time, which aired last night in the United Kingdom. I'm steeling myself to be utterly heartbroken after tonight's installment, which features the return of John Simm's malevolent Master and several familiar faces, including Bernard Cribbins' Wilf and Catherine Tate's Donna Noble.

But I can't help but think back to when I first heard that Christopher Eccleston was departing Doctor Who and would be replaced by the relatively-unheard-of David Tennant, whom I recalled slightly from his turn as the titular character in Casanova.

At the time, I was deeply saddened that Eccleston was leaving Doctor Who and would be replaced by someone who seemed so completely different to him, one whose Doctor wouldn't be the same as the gruff, muscular, and stolid Doctor embodied by Eccleston. "Who," I asked my wife (then my girlfriend), in a unintentionally ironic question.

What a difference a few years make. Tennant--with his manic, madcap, and mischievous take on the Doctor--has become in my mind the definitive performance for the Time Lord and now he too is stepping aside as another relatively unknown actor, Matt Smith (Party Animals), replaces him after a roughly four-season run on the series.

Will Smith supplant Tennant in my esteem? Time will tell, though he has some mighty big shoes to fill. But as we settle in tonight to say goodbye the talented Mr. Tennant (and to executive producer Russell T. Davies, also departing), my thoughts won't be of the future of the series--of Matt Smith and new head writer Steven Moffat (both supremely skillful men in their own right)--but of the past. I'll be recalling Tennant's legendary run on the series, the off-kilter way he dove into every new adventure with equal parts glee and curiosity, the times his Doctor saved countless people and the times he failed, and of the actors who played his companions these past few years: Billie Piper, Freema Agyeman, and Catherine Tate.

The Doctor and Doctor Who are bigger than any single actor, yet Tennant's performance, which captures both the innate strengths and flaws of the alien time-traveler, will remain a highlight of the decades-old drama. As we prepare to embark on one final trip in the TARDIS with the unnamed man from Gallifrey, I find myself both tearful and exhilarated to see just what will happen next.

The Doctor, I am sure, wouldn't have it any other way.

You can read my cocktails-laden interview with David Tenannt from a few months back here... and watch my video interviews with Doctor Who executive producer Russell T. Davies and director Euros Lyn here.

Doctor Who: The End of Time, Part One airs tonight at 9 pm ET/PT on BBC America.