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It's not exactly the end, but it's the start of what will be the end of one popular aspect of "Doctor Who."

And it's no secret: David Tennant is leaving.

This weekend, British television viewers will have a chance to see how this endgame will start with the special "Water on Mars," which airs Saturday on BBC (and coming to BBC America in December for American viewers). Tennant has wrapped the series for some time, but even when he sat down with a group of reporters including Airlock Alpha during San Diego Comic-Con in July, he missed being part of the "Doctor Who" family, but still excited about what fans would be seeing soon.

"There are such high points to come," Tennant said. "Some of our strongest work is yet to come."

For "End of Time," which airs Christmas Day on BBC, Tennant said actor Bernard Cribbins, who plays the character Wilfred Mott, has a "huge and fundamental part in this final story." At the same time, he can't wait for Catherine Tate to return, because having "a little bit of Donna is never a bad thing."

And of course, the return of John Simm as The Master, a role that was finally confirmed by the "Doctor Who" cast and crew during Comic-Con.

In this weekend's special, a mysterious water creature starts inhabiting people on Mars, and The Doctor has to stop it. Among the actors making a guest appearance is former 007 Timothy Dalton.

Tennant's portrayal of The Doctor has been an interesting one, and a darker one than many fans were familiar with. At the same time, many have called Tennant their favorite Doctor.

"I think he feels guilty," Tennant said. "He is in a very difficult position. He has to make the hard choices, and he is riddled with remorse to what happened to his people and the part he played in that."

And about that, Tennant did tease that more details about what exactly happened to the Time Lords will come out during these final specials, at least before he departs and Matt Smith takes over.

"Not that much, just a little," Tennant said. "It won't be a three-part miniseries starring Paul McGann."

Tennant couldn't announce what he was working on at the time, but since, he's been attached to an American series where he plays the title role in "Rex Is Not Your Lawyer." That show is in pilot stage, and there is no word on if NBC will pick it up.

"Waters of Mars" airs Sunday on BBC and Dec. 19 on BBC America.