David Tennant ends his reign as the Doctor in "The End of Time," his final episode of the BBC television series Doctor Who.

Since my childhood, I have followed the adventures of the Doctor, a renegade Time Lord who travels through time and space repeatedly saving the universe and (more often than not) the planet Earth. So, tonight’s episode “The End of Time – Parts I & II” (on BBC America) is more than an event, it’s a goodbye.

For four years now, the titular role on the BBC television series Doctor Who has been David Tennant. With a variety of companions and friends, the Doctor has met threats from outer space and beyond. Now, everything comes full circle as he encounters his arch-nemesis, the Master (John Simm), and the reemergence of the believed-extinct race of Time Lords.

“The End of Time” is not just the end of a series. It is the end of Tennant’s tenure as the Doctor. With this episode, he leaves the series to a new actor and a new doctor, the young, emo-looking Matt Smith. So, the story has brought more than an anticipation of something big, it has brought a sense of loss as one of my favorite characters prepares to change once again.

Watching Doctor Who over the years, from the days of Jon Pertwee in the early ’70s through the present, I have discovered that the only thing consistent about the Doctor is change. As often (if not more) as the Doctor regenerates, his companions have come and gone. His villains have been born, died, and been reborn again. Even the Time Lords, the race of time travelers from which the Doctor came, were wiped out in a Time War with the Daleks. The only thing that has remained unchanged over the years is his TARDIS, a time-traveling ship forever frozen in the shape of a London police call box.

Despite the changes over the years, I find myself unwilling to let go of Tennant and his version of the Doctor. The past four years have given audiences so many unforgettable moments, including the return of the Doctor’s companion Sarah Jane Smith (Elisabeth Sladen) and the introduction of Captain Jack Harkness (John Barrowman). Both Sarah Jane and Captain Jack were quickly spun off into their own series, The Sarah Jane Adventures and Torchwood, respectively.

Tennant’s Doctor has been blessed with some amazing companions, including Rose Tyler (Billie Piper), Martha Jones (Freema Agyeman), and Donna Noble (Catherine Tate). Each of his companions brought a different flavor to the stories, with Rose as a love interest, Martha as an unrequited love, and Donna as a strong-willed best friend. The importance of his friends and companions was made most clear in the fourth series finale (the two part story “The Stolen Earth” and “Journey’s End”). In that story, each of his friends comes together across time and space to help repel a Dalek invasion and save the Earth. It was, for me, the finest story of any Doctor Who.

But in his last year, through the final four specials leading up to and including “The End of Time,” the Doctor has been traveling alone. Without a human companion, he has become more lonely and — as a result — more Time Lord than man. He has become arrogant, godlike. The effect, seen most clearly in “The Waters of Mars,” is almost frightening.

“The End of Time” returns the Doctor to Earth. It also reunites Tennant with his human companion Donna, by proxy through her grandfather Wilfred Mott (Bernard Cribbins). Though the Doctor has returned to Earth (after the Master’s return is foretold by the alien race The Ood), it is Wilfred who seeks out the Doctor. As the human element of the equation, Wilfred gives the Doctor a touchstone — something to anchor him through this dark time when (as the Ood prophesied) the Doctor’s “song is ending.”

Even if the Doctor regenerates, a part of him dies with us. Tennant is leaving, and with him goes some of the best moments and memories of Doctor Who.