Like the best science fiction, Doctor Who is strongest when it uses the possibilities afforded by its imaginative nature to explore the triumphs and shortcomings of humanity in a deeper context.

Earlier this week showrunner Steven Moffat revealed Saturday's episode "Thin Ice" would tackle the subject of race head on, including the fact that history is frequently white washed, as part of Bill's (Pearl Mackie) first trip into the past. What he didn't reveal was how the episode would touch on the Doctor's (Peter Capaldi) own difficult past, Bill's reaction to it and whether it matters.

In the episode, Bill and the Doctor travel to 1814 London, where an alien creature is chained under the frozen Thames and being exploited by a single family for selfish gains. Addressing the monstrous man behind the entire operation, the Doctor did what the Doctor does so well, which is deliver the kind of speech we all wish we could in moments where we're tested.

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"I preferred it when you were alien; that explained the lack of humanity," said the Doctor. "What makes you so sure that your life is worth more than those people out there on the ice? Is it the money? The accident of birth that put you inside the big fancy house? ... Human progress isn't measured by industry. It's measured by the value you place on a life, an unimportant life, a life without privilege. The boy who died on the river, that boy's value is your value. That's what defines an age. That's what defines a species."

TVGuide.com recently got Moffat on the phone to talk about this powerful episode, the mysterious vault, the relationship between the Doctor and Bill, and all of Bill's amazing questions. Read on to see what he had to say.

It might have been all of the snow, but I got a little bit of a feel of the Christmas episodes there. What was the idea going into this episode? What were you trying to get out of the Doctor and trying to get out of Bill?

The idea of the episode was brought to me by [writer] Sarah Dollard because I was saying, "I'm in the market for something where the Doctor takes Bill to the past. Have you got any ideas?" She came to me with the Frost Fair idea. It came from that.

Throughout the early part of the series, probably through to the end of the series, I am always wanting to be clear on every script and every scene, what new thing is Bill learning about the Doctor or the universe? What new thing is happening to her? What is a first for her in this episode? How, when she goes to bed at night, has she moved forward or moved backward? How has she become more scared or more confident? What has shifted inside her by the events of this story? These are really quite traditional Doctor Who stories, but we can sharpen them and make them land harder by remembering the stages of someone learning. It's not just this week's adventure for clever old Doctor Who.

There are some new monsters, or creatures let's say, in the episode. What was the inspiration behind them?

The title sequence of Eastenders. It does look like a big snake, the Thames. I can't remember who said it. It might have been me, but it was, "If we're going to do the frozen Thames, have you seen what the Thames looks like?" It's a big squiggly snake shape. That would be cool. That's a bit vague, but something like that.

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Let's talk about the very end of the episode for a moment. We've gone back to this vault a couple of times. Is that a season-long arc we're going to find out more about toward the end of the season or is that going to be revealed somewhere toward the middle?

I'd rather let that play out. I'll put it this way: I'm not going to put any dates on this. The mystery of the vault doesn't stay static. You learn things. Things happen. There are surprises about it. It's not just that we have a closed door and the Doctor wanders by every so often and thinks, "I better keep that shut then." There is progress, but I'd rather just tell the story in the right order if that's okay.

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The relationship between Pearl Mackie and Peter Capaldi, and Bill and the Doctor as well, is fascinating and very different from the previous companion relationships, at least for this new series. They very much have a teacher/student relationship. How did you come about that idea?

I remember that we had several meetings about the new companion, the new series arc and all of that. ... Because we have the Peter Capaldi doctor, [it's] a relationship between a much older man and a much younger girl. What do we normally call that? Well, we don't really want to call it the aging rockstar and his groupie. That would be horrible. What else could it be? It can be father and daughter, absolutely could be that. Although fathers and daughters don't tend to run off and have adventures together. You know, teacher and student. Educating Rita — the old film and play Educating Rita — I thought was kind of a good starting point for that kind of relationship.

It has all the spark and joy of a non-romantic romance. Do you know what I mean by that? When you meet somebody who really does become incredibly close to you and it has an awful lot of what a romance has, except it doesn't have any sex or romance in it. It's just got, "Here's this person I've got this different kind of crush on." When you meet the people in your life who do become your great teachers, you do have a different sort of crush on them. As I say, it's not an erotic one. It's a fascination. It's a joy in their company. It's meeting someone who opens new doors to you. That's a good relationship to model the Doctor and his best pal on.

If the Doctor is teaching Bill, at a certain point teachers have to let their students graduate. What is the Doctor preparing Bill for?

He's really not preparing her for anything. He hasn't got an evil plan here. He just misses someone like Bill to talk to. That's what he gets out of it. He's not training her for something. I don't think he ever thinks that way. There's just a whole lot of universe out there. There's a whole lot of fascinating, exciting stuff out there. You're going to the same place every day and selling chips. That's a waste! Come with me and I'll show you stuff. He doesn't have an end plan in that. If you told him that he was just teaching her, he would probably say that he learns as much from her as he ever teaches her. He has a lack in his life, too, which is the ability to be awestruck, the ability to be emotionally committed in a different kind of way. He misses all of that and needs it.

You talked about this a little bit before the season premiered, but Bill gets to ask all of these questions that you just don't ask about Doctor Who at this point. They feel new and fresh in a way. Do you have a book somewhere where you've written down all of these questions and decided to roll them out of Bill?

I don't know why this has come up. It was my first idea. What if somebody just asked the question that so many people don't ask in Doctor Who. It's scraping stuff out of your own fan brain and the assumptions that you make. For instance, people walk into the TARDIS for the first time and of course they know what to do if they go on the inside. If it's standing against a wall, your first assumption isn't that it's bigger on the inside. Your rational assumption would be that you've stepped into another room. More critically, they always say, "It's a spaceship." How the hell would you know unless you know that that little room with no chairs in it and a work station in the middle was a space ship? How would you know? It doesn't look like the Enterprise at all.

It was refreshing to rinse your brain out and say, "Really, really start from the beginning." There's so many things that we've accepted over the years that there is no rational reason to accept any of these things, including the the fact — there's kind of a statement of intent at the end of the very first scene Bill's in, where she says, "Doctor what?" instead of "Doctor who?" because that's actually the more common expression. She doesn't even know to say, "Doctor who?"

Doctor Who airs Saturdays at 9/8c on BBC America.

Additional reporting by Alex Zalben