Fans of David Tennant's portrayal of the 10th Doctor have only a few more chances to see him in action before he morphs into Matt Smith, and BBC America previewed two of the final Doctor Who specials—"The Waters of Mars" and "The End of Time"—Wednesday in a press conference in Pasadena, Calif., as part of the Television Critics Association summer press tour.

The trailer for "Time," Tennant's final Christmas special, offered very little in the way of details. Timothy Dalton's voice-over intones: "In the final days of planet Earth, everyone has bad dreams." Beyond that are simply a series of intense shots of the actors' faces—including a quick shot of Catherine Tate's Donna Noble and a final shot of ... John Simm as the Master? ... and the words: "He returns." Davies was cagey about it all, including Dalton's involvement. (This is the trailer that screened at Comic-Con.)

David Tennant

"He's playing a character called the Narrator, but you'll have to see more at Christmas for that," Davies said in a group interview following the press conference. "He's wonderful, that voice."

Davies would not confirm that Dalton would appear on screen, but enjoyed repeating that his character's name is the Narrator. Even choosing clips for the teaser was a debate.

"We argued over them long and hard, because there's an editor called Thomas back in BBC Cardiff who put those together," Davies said. "We went over them 27 times, every time saying, 'No, take that out. No, take that out.' Because I know science fiction fans are very clever. You can show them one character, and if he's hiding something in his hands, they will guess what it is. If he's hiding something, they will guess why he's hiding something, and therefore they'll guess what the entire plot is. I've seen them do that. So it's such a literate audience, all you really get in those Christmas clips are faces, to show you who is in it, because we daren't show any more."

As this will mark Tennant's last appearance as the Doctor, longtime fans expect to see him morph into the new Doctor at the episode's end. Or will he? Davies hinted that he might turn tradition on its head. "Maybe," he said. "That's what typically happens, but maybe it'll be different this time."

"Waters of Mars" was a bit easier to discuss. The story involves a viral outbreak on a Mars station due to contaminated water. Davies conceded that "Waters" is about a base under siege by monsters.

"It's got many templates," Davies said. "It's fun stories, and it's dark stories, trapped in the base. It's what Doctor Who fans call the base under siege. Actually, in that story, it's under siege from within. It's what I call [a] submarine job. It's just trapped in one space. Doctor Who didn't invent that. Having a unity of time and space is a great engine for any drama, but it's really effective when there's a monster on the loose. It's twice as effective then."

Even so, Davies has not given away everything. "'Mars' is much more a classic story as well," he said. "There are monsters, they're on Mars. There's a secondary story underneath that of what the Doctor goes through. It's a more subtle story. It's monsters on the loose, but there's a great Doctor story underneath it all. So even that, you haven't quite guessed from seeing it. You'll never guess the ending of it."

With a successful run reinventing Doctor Who, Davies is ready to go out on top. "Imagine getting bored with it," he said, explaining his decision to pass the show-running torch to Steven Moffat. "Imagine it going off the boil/bottle. Also, David Tennant's leaving it in good health, because a new team will take over. They'll fly with it. You just watch."

Fans can rest assured that Davies is leaving Doctor Who in good hands. "It's in great shape," he said. "It's the number-one show in Britain, not just the number-one drama. It's the number-one program, ahead of everything else. So it's both in great shape and it's a clean slate for them. So they can do whatever they want."