Once a cult series, British sci-fi drama Doctor Who has become a global phenomenon, and new audiences are embracing the 900-year-old alien time traveler—now played by 29-year-old Matt Smith—with alarming passion. (Witness the rock-star welcome Smith and co-star Karen Gillan got at July’s Comic-Con.)

Doctor Who, under head writer Steven Moffat, who replaced Russell T. Davies last season, returns for the second half of its sixth season in the U.S. and the U.K. on Saturday. The Daily Beast sat down with Moffat in Los Angeles to discuss the shocking identity of River Song (Alex Kingston), criticisms of “bad girl” companion Amy Pond (Gillan), and next season.

It was revealed that River Song is the daughter of companions Amy and Rory (Arthur Darvill). Is this the solution you had in mind when you first introduced her in “Silence in the Library” back in Season 4?

Not quite…It came about from the most practical reason: I had to have the Doctor in a library…and a bunch of archeologists had to find him and not immediately arrest him for the crime. The psychic paper wouldn’t cover it, so I thought, What if one of the archeologists knows him?...But then I thought, What if one of the archeologists knows him, but he doesn’t know the archeologist? Suddenly, that’s quite cool. What if it’s a woman? What if it’s a woman who flirts with him in a rather proprietary way? Then a whole story explodes in your head…She might be his wife, or a girlfriend, or there’s a romantic attachment there.

When I introduced Amy Pond, it was with the possibility that this could be the mother of River, and that’s why I put the “Pond” in…There was no guarantee that Karen [Gillan] was going to stay or that Alex [Kingston] would come back. It was just a possibility that I kept alive.

What can you tease about the rest of the season, beginning with “Let’s Kill Hitler”?

It’s an absolute cracker…I can’t tease but I can give you some promises. Answers will be given. We’re not playing at really being Lost. You will know what’s really going on on that beach, you will know the truth about River Song, which you don’t know yet…It’s pretty much answer, answer, answer in a way...But by the end of episode 13, new things will have begun...The great thing about the River Song conundrum is that every time you get an answer, it makes you ask another question.

There have been reports of budget cuts being behind the BBC’s decision not to air a full season of Doctor Who during 2012—

Absolute nonsense. First of all, we are airing in 2012. The only thing that’s happening is that we’re moving a bit later...There’s lots of reasons for that that will become clear quite soon...It is certainly not a reduced episode count. Do you think the BBC would really let that happen? With an average audience of 10 million?…Doctor Who’s international profile is huge. It’s never been more successful. You’re not going to reduce a show like this. The opposite is going to happen, in fact.

Eventually, you and Matt Smith and Karen Gillan will—

Die?

—move on to other things. Do you have an exit strategy for Matt Smith already planned?

In the vaguest terms, I’ve already thought, what would be the last stand? What would be the most heroic way to go? Which I might use when that terrible day comes. But not an exit strategy. I don’t want to think that the stories are finite; I want to feel that they can go on forever.

You’re splitting your time between Doctor Who and Sherlock. How long do you foresee your tenure lasting on this series?

I’ll run out of stamina before I run out of ideas is the absolute truth, because I am living a life without a single day off...So, at a certain point, that may prove to be too much. I’m not feeling that right now. I love doing it. I slightly dread the idea of leaving, to be honest. It would be really hard to feel that the Doctor isn’t mine anymore…That day did come for Russell, and he did say to me, because I was asking how he felt, “I love it as much as I ever have, but I just don’t want to do it anymore.”

Neither of us would know ourselves if we didn’t love Doctor Who. That would be so terrible to lose that…The same was true for David [Tennant]. I had to deal with him during that time, he’s a friend.

How far ahead do you plot your Who stories?

Four pages [Laughs.]

Clearly, you have a season-long arc in mind. Do you look at it as season to season?

[Season to season], because you never know what’s going to happen and you want to be allowed new possibility. I always tend to favor the newer idea…I was saying to my writers, “Don’t think of your next episode yet: you’ll be bored of it by the time you get there…” Obviously, this year I had to think about it a little more carefully, because I knew what we were going to do with River, half of which you’ve already seen. This is the most arc-intensive we’ve ever been and I’m throwing the lever the other way next year.

How would you stack up Amy Pond against her most recent predecessors, i.e., Rose Tyler, Martha Jones, and Donna Noble? Did you set out to make her a reaction to any of the Doctor’s most recent companions?

I don’t think you can, because she’s a completely different person. With the Doctor, you might have a certain amount of that, but Amy Pond has never even met those other girls, so why would she be a response to them?…There are always going to be certain commonalities to the people who choose to go through the blue doors, but I think Amy is…in certain respects, one of the trickier ones. Because of her very odd introduction to the Doctor, it took her a long time to trust him. She likes a good time…I keep saying, “bad girl in the TARDIS.” And now you know who her daughter is, and they’re both just bad girls.

Some people have gone so far as to say that Amy is a plot device more than a character.

I never heard it said, so I don’t know how to respond to it…But she isn’t. She actually gets in the way of the plot sometimes. Amy’s tricky—she doesn’t always do what she’s told, she’s not quite as adoring of the Doctor as she ought to be, she’s naughty.

Part of the criticism comes from the fact that Amy is purely defined by her relationships with the men in her life—the Doctor, Rory—as opposed to her predecessors, for whom we saw vibrant, active home lives, aside from any romantic attachments they might have.

We see an active home life for Amy. Her boyfriend and her…I don’t see what’s different there.

Well, any family life for a start. Or life outside of Rory.

We don’t see her mum and dad much…[But] I can’t think of anyone who isn’t defined by their relationships…I think she’s sometimes defined perhaps by her problematic relationships with those people, her slight indecision about who she’s in love with, her willful selfishness at times, and her badness, which I quite like. As I say, “bad girl in the TARDIS.”

How has the dynamic changed within the TARDIS to have a married couple along for the ride?

In very fun and interesting ways…The big thing with the Doctor is that he thinks he’s…intruded for too long. Each time he tries to extricate himself, there’s another complexity that means that he can’t. Suddenly, she’s married. Her husband-to-be is dead, then he’s suddenly back again, he’s on their honeymoon, he’s dating their daughter. It’s complex and that’s what he’s always thinking: when is he going to get his exit? He has an exit strategy for all of these relationships, including Rose, whom he loved so much, because he knows he can’t hang around, he’s going to cause too much damage. In one of the upcoming episodes, he sits in this room and says, “I can’t keep doing this to them.” It’s too much, it’s too deadly.

Should we find that relationship strange, that the Doctor is dating their daughter?

If he is indeed dating their daughter. He seems to have, at least if nothing else, a woman who flirts with him with a wifely confidence. How will that work? I think Rory will probably punch him twice a day: “Hands off both of them!” I don’t think it will bother Amy for a heartbeat because actually she has made a choice. She doesn’t want the Doctor. Or rather, she’s got the Doctor in that way and she would think, “You go, girl. Go get him!”

We had Neil Gaiman’s episode (“The Doctor’s Wife”) this season. Is there anyone else on your dream list to guest-write an episode?

The truth is the less experience they have in television, the more you have to rewrite their script, so that can be tough...All the people who are frequently cited as “you must get an episode from so-and-so” haven’t written a television script. However brilliant his script, I will end up writing 90 percent of it and I don’t have the time...Doctor Who defeats some of the most hardened television professionals ever. I’ve been working in television for a quarter of a century, and it’s the hardest thing I’ve ever done.

What is the key, then, to a perfect Doctor Who episode?

You have to juggle comedy, you have to juggle adventure, you have to be good with pace: move things really fast and understand that it’s not about working with 50 lines of dialogue but getting the hell on with it. You have to know how to write to savage limitations but make it look like a movie by being very clever and judicious with your sets and your locations and your CGI…I think Doctor Who is a pretty bloody lavish show, but it really is not. It’s an exercise…in clever exploitation of limitations.

Have you seen the American broadcast’s pre-cold open sequence with Amy Pond? How do you feel about that?

I think it’s a bloody good idea. I think it makes it accessible—I know the diehard fans don’t like it…It’s stating the bleeding obvious for them, but for a brand-new audience, that’s a way in. Oh, I see, she’s human, we go in the TARDIS, we get it. It’s strictly for the foreign [broadcast], it’s not on in Britain. We were asked to do it; I wrote it. Actually, the person who said, “I bet everyone else hates it, but I absolutely love it” was Russell…If you don’t like it, ignore it, go and get the tea…It’s the right decision, and I know it’s the right decision because the ratings are 20 percent up. That’s huge.

To what do you attribute that?

If you happen to catch it, it’s a very good show. Even if you don’t understand it, it’s a very good show. The ones we’ve been doing—David’s, Chris [Ecceleston]’s—you watch it and think, that’s bloody good, I’m going to watch it again. There’s been an accumulation of that going on, reaching an incendiary point. The promotion has been really, really useful. And because Doctor Who renews itself every now and then, you can say, “I’ll start with him then. He looks nice. I’ll just pop in and start watching here.” Otherwise, you’re going to have to start watching in 1963, with [William] Hartnell, and that might be a bit of a trauma.

Editor's Note: An earlier version of this story incorrectly identified River Song's first appearance as occurring in Season 5.