Warning: This article contains BIG spoilers for Humans series 3, episode 8

So, Humans went there – where we feared it might, and hoped it wouldn't.

The latest series of Channel 4's spectacular sci-fi series ended with the devastating demise of Mia – acting as a martyr for the Synth cause, Gemma Chan's character was brutally beaten to death by bigoted thugs, live on television.

The shocking scenes marked Chan's departure from Humans after three years, with the actress telling Digital Spy, "She's been on a huge journey, this character, and it's been a really interesting ride.

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"We're a close team and it's been very exciting to be part of something that feels relevant, and that people have really responded to. People come up to me, and it might be a kid, or it might be a grandma – I don't think there are that many programmes that appeal to that wide a range of people."

Of course, Mia's death was just part of a wild finale that also saw Niska (Emily Berrington) evolve into a higher being and Mattie (Lucy Carless) discover she was carrying a human/Synth hybrid child.

To discuss all this and more, we caught up with Humans creators Sam Vincent and Jonathan Brackley for an exclusive deep dive into series three, episode eight (written by Daisy Coulam, and directed by Richard Senior).

There's only one place to start, really... When did it become clear you would be writing Mia and Gemma Chan out of the show?

SAM: Relatively early on. Obviously Mia is a completely central character to the show, and we took her on this journey – we were plotting her story, and it became clear that she was going to become this political symbol, a lightning rod, a face around which a group can gather.

And it was going to be about sacrifice. I remember, when we talked about her character, she's always been extremely self-sacrificing. She's always put others before herself, from Leo (Colin Morgan), and being a mother figure to him, to giving her life for Laura (Katherine Parkinson) in series two.

And we sort of felt like that self-sacrificing quality is the essence of the character in so many ways. And when we started her on this path, we felt that this could be the culmination of all of the stories of series three in many ways, in that every great movement, you could argue, needs a martyr.

I mean, I don't necessarily agree with that, but many great movements have pivoted around a martyr. And when we had this idea that she could actually quite consciously make the decision to put herself in harm's way because she understood the power of the world seeing that.

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We felt like it was really exciting, because it was true to the character. It was meant to be moving, but it also was about how sophisticated and intelligent she was; that she makes a conscious choice to put herself directly in the firing line, because she knows the power of that image – that she had that kind of political savvy was really thrilling to us.

And obviously it was an extremely big deal to even contemplate killing off a central character like this. We spoke about it. Internally, when we were sure that this was the right story and felt true, we went and spoke to Gemma about it.

It was obviously extremely important to talk to her as early as possible, and to bring her into that process, and get her thoughts on it.

So it was a pure storytelling decision, then? It wasn't motivated by anything else?

JON: No. It was purely borne out of storytelling decisions, from the very beginning.

Did you weigh up any other possible ways of writing the Mia character out?

SAM: As I say, we didn't have any authorial imposition to write the character out, so we considered multiple other options.

Essentially, I think everybody felt that nothing was going to be as powerful and as true to the spirit of our story as this. So I think we certainly considered, "Could we be ambiguous about it? Could we do a twist? Could we end on a mysterious, leaving the possibility open for her return?"

But ultimately we felt that to do something like that is just hedging your bets, and you're not really committing to the story that you're telling.

JON: And ultimately, it weakens the power of what we're doing. It dilutes it somehow. It's kind of a cop-out, really – a storytelling cop-out. To have real power, you've got to be bold and definitive.

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Is there anything you wanted to say about Gemma, and about her contribution to the series?

SAM: Well, we wouldn't be talking about season 3 if it hadn't been for Gemma, because she made that part her own. In season one, her depiction of Anita and then Mia was absolutely vital and crucial to the success of the show. It simply wouldn't be Humans without Gemma. She's been the face of the show.

A little story that we could share with you is, Gemma was actually the very, very first audition that we ever saw on tape. We weren't present in the room, me and Jon, but she was the very first video audition that we'd seen for anybody for Humans.

We watched it, and Jon and I just looked at each other and thought, "Oh, this is going to be easy." Because she had an instinctive understanding of the character. She has the talent and intelligence to play two parts in one in that first season, and to carry the whole thing. So yeah, it wouldn't be Humans without Gemma.

Once you knew that you would be writing Mia out, did that impact on Niska's arc for this series? Did her arc then become all about making her ready to become a leader?

SAM Yeah. That was very much in our thoughts. We're always trying to do something new with the characters, and push something challenging on them. And for Niska, she has this great responsibility now. Literally, in episode eight, we see her say, "No, no, no – you're thinking of Mia. She's the mother of her species. She is the leader of the Synths."

But for that baton to be passed onto somebody who's not ready for it, or isn't quite suitable, or is going to struggle to don that mantle... we felt that that was a really interesting point of connection between the two of them. And we loved that moment.

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What exactly are Niska and V now that they've evolved into "Purple Eyes"?

VINCENT: Well, as the fans will know from season two, V is a near-omniscient AI programme that started off as a kind of template of the human mind, but has evolved and grown into something much more.

V isn't limited by a body, and has this omniscience because she's everywhere on the internet, and is connected to all things, and essentially, V has looped Niska into that.

So Niska now has a kind of similar all-seeing eye, and other abilities besides, perhaps, that we wanted to keep in our back pocket.

Let's talk about Mattie, who's giving birth to a human/Synth hybrid baby. What inspired that idea?

SAM: That child was referred to – and I can't even remember why now, but throughout the whole story-lining process, that child was referred to as "the turbo baby". I think it might've been our producer, the wonderful Vicky Delow who actually coined the term!

We've always had in the back of our mind that eventually there would be a synthesis – no pun intended – between humans and Synths, and that they would come together in some way. The great truth, the answer that we wanted to have, is that we're not different to them and the future relies on mixing and melding [with] them.

It of course pushes the boundaries of just how science fiction we can be within the parameters of this show. But we wanted to find a way to examine how Synths and humans can come together.

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Of course, we've had it before in Leo, who's a central character. But when we struck upon this, we thought: it's quite momentous, but it's also very, very human, because it's a young woman who's fallen pregnant, with potentially extraordinary stakes.

We knew that we were going to have Mia's death, and this huge battle, and the scenes with Niska and Odi, so we wanted to play all of that huge stuff, and then come back down to one of the most human things we have, which is Lucy Carless's face, as she hears some impossible news.

We were always keen to have that as the final shot of this season. And fortunately, we were able to make it work, courtesy of Daisy's writing and Richard's great direction in this episode.

How far ahead are you in thinking about a potential fourth series of Humans, and the themes you might explore?

JON: Well, we definitely have a story to tell, if we're lucky enough to get a fourth series.

As ever, as we're writing the third series, we can't help but think about what that fourth series would be. Because of the events at the end of episode eight, again, we're going to be coming back to a changed world. We join a very different world. The world is going to evolve again into series four, if we're lucky enough.

SAM: Yes, it would be quite different. We know what our theme is, but I don't want to say it just now. We're excited about the ideas that we have, so we just have to hope, really, and do our best to communicate those ideas to the powers that be.

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