Derry Girls returns to Channel 4 on Tuesday at 9:15pm for its long-awaited final season. Below, writer-creator Lisa McGee lets fans in on the end of an era.

Where do we find the girls in this third series?

Lisa McGee: What we've tried to do this season is to have a theme and a genre in every episode, like a little movie. Episode one takes its tonal inspiration from Goodfellas. We meet the gang on the eve of them getting their GCSE results, the episode is called The Night Before, and it's all set over one evening. The girls are worried they're going to have to run away from home because they're going to be so bad, and wind up getting arrested. Meanwhile, Grandpa Joe's taken in a stray cat. He's really fond of it, but it's killing all the animals in the neighbourhood, and him and Gerry have to dispose of the body of a pet rabbit. So it's a bit like a very silly thriller, very moody and sort of noir.

In episode two, the family hire a hot plumber who everyone starts to believe Ma Mary's having an affair with. It's fun trying to work out what's actually happening. The gang are also going to perform in a Stars in Their Eyes concert at school for Children in Need and those two stories crash together.

Episode three is called A Stranger on a Train. So the whole episode is set on a train, and there are two different stories that cross over as the kids and adults are sitting in two different carriages.

The Night Before

Episode four is a kind of horror film where the girls end up going to this creepy house in Donegal and lots of weird stuff happens. Meanwhile, the family go to see a psychic to try to connect with their dead mum. It's a really lovely episode, and one of my favourites.

Episode five is something very different for us. Ma Mary and Sarah are going to their school reunion, the class of 1977. We learn that something happened at that school reunion years ago that was like I Know What You Did Last Summer, but not anything actually that bad!

Episode six is Halloween. Halloween in Derry is massive anyway, but the girls are off to see Fatboy Slim. They don't get tickets, so Michelle tells this elaborate lie on national TV in order to bag some VIP passes. Obviously, they get caught, and that has lots of consequences. It's a really joyful, lovely episode. Clare meets a girl...

They've had a naivete up until now, but this series leads up to the Good Friday Agreement. Does that make them grow up a bit?

Yes, they've never had to really think about politics before. What the Good Friday Agreement did for my generation was made to [sic] think about things like, 'Do you think paramilitary prisoners should get out?', 'What do you think should happen to the police force?', 'Should the army be there?' These were all big questions that you had to have an opinion on. But the girls do have to start taking responsibility for themselves, and the vote was a really good way of showing that responsibility. It's the moment you feel like a grown-up, when you vote, so I'm very proud of that episode. They also have a huge falling out because they don't agree, they don't believe in the same thing. They get to a point where they go, 'What are we going to do?' Life's not that simple anymore.

"What we've tried to do this season is to have a theme and a genre in every episode, like a little movie"

Have you written in the grown-ups a little bit more this season?

Yes. Ma Mary, in particular, has a couple of big moments. I was almost being led towards her as I was writing it. What I love is that Aunt Sarah and Ma Mary have their own stuff that they get up to that nobody else knows about, their own little schemes. Gerry and Granda Joe go on a couple of mini-adventures and things too, so it's been lovely developing those stories out a bit more.

And what about Sister Michael?

Her and Father Peter are involved in a number of altercations, but then at the end he's kind of forced this friendship on her, and there's this really moving couple of scenes between them. And you realise what he actually means to her, and that he's an idiot, but he's really a good person as well. He cares. Siobhán [McSweeney] has done some incredible not just comic acting, but character acting too. She just understands that character so well.

Get ready for "a number of altercations"

How does it feel putting Derry Girls to bed after all this time?

It's emotional, because it's become a bigger thing than I ever thought it would be. I don't just mean as a piece of work or whatever, but for my city as well. It's been this amazing, massive thing in my life for more than six years now. I read one of those platitudes people put on Instagram that said, 'Don't be sad it's over, but glad it happened' and that's how I feel. I am so grateful to have been able to do this in the first place, and the fact that I've been allowed to finish it the way I wanted to finish it has been incredible. I don't think many writers can say that, that they've been given the chance to really say what they want to say, the way they want to say it. It's such a privilege and honour, really. I'm a bit nervous about some of it, but I can't wait for everyone to see it.

Which bits are you nervous about?

Oh, God, always anything that touches on The Troubles. It's normally fine, and I've got away with it so far, but I never want to offend anyone. I want the show to bring joy, but to be real, so I have to tackle it. I just hope I get the right side of the line.

Lisa McGee at the season three launch of Derry Girls in the city earlier this month - "It's become a bigger thing than I ever thought it would be"

The global response has been phenomenal. Why do you think that is?

I was thinking about this recently. I always remember watching Friends when I was 15, and Chandler said, 'You stop the Q Tip when there's resistance'. I laughed, even though I had no idea what it meant other than he was saying Joey was stupid. I didn't know what a Q Tip was. But you feel the rhythm of it and understand that it's funny. And I think that's what’s happened with Derry Girls. It doesn't matter if people in other countries don't know every nuance.

What is it like when you hold a premiere or an event in Derry?

It's incredible. It's hard to really put that into words, but this will never happen to me in my career again. But what's also funny is I've moved to Belfast, and my name is recognised. One of my heroes is Jessica Fletcher from Murder, She Wrote and I don't know if you've watched the show but occasionally people say to her, 'Are you THE Jessica Fletcher?' So now, my favourite thing is when I'm picked up by a taxi and the driver says, 'Are you THE Lisa McGee who wrote Derry Girls?' [Laughs] I'm like, 'I'm the Northern Irish Jessica Fletcher!' It's the coolest thing. I love that.

"It's going to be weird not talking to them for a while"

Do you ever think about what the Derry Girls would be up to in 10 years’ time?

I have! It's so weird because I think of them as real people. I actually got so worried about Orla at one point that my husband was like, 'She’s not real!' I think a lot about what they're all doing. I'd love Ma Mary and Aunt Sarah to solve a crime. I decided that Orla is probably coaching a really s*** sports team, like the Mighty Ducks, and she leads them to glory. Michelle's a DJ. Erin's a novelist. Clare's a top barrister somewhere. And James is a filmmaker.

He's respected at last!

Yes, he's very good, actually. In this season his mum sends him a camcorder and he starts making films, so you see the beginnings of that.

I love those characters. They feel like real people to me, so it's going to be weird not talking to them for a while because I've been talking to them every day since we started. It's going to be strange.

Is that how you work? Do you literally have conversations with them?

With myself, yes. Walking around the house. Sometimes walking down the street in public. I always have my headphones on in case someone thinks I'm mad because I'm talking to nobody.

"I'm a bit nervous about some of it, but I can't wait for everyone to see it"

What was it like with the cast when you finally finished filming?

We were all really tired and emotional. The last thing we did was in the school. It was so crazy to finally be at the end of this incredible journey, which was tough and joyful in equal measure. The production got everybody a painting of Derry, which was really gorgeous, so I have that in my house.

Did you take any souvenirs home with you?

No! Everyone else was doing that, and I didn't! I really wanted the Angela Lansbury poster, the Jessica Fletcher poster, but I didn't get it. I might ask if it's still there somewhere. I never got time. I'd sometimes go to steal something and then get distracted by some question about something.

"I don't think many writers can say that, that they've been given the chance to really say what they want to say, the way they want to say it"

What do you think the secret to its popularity was in the end?

Maybe that it's just joyful, and you're going to be cheered up, and it's nostalgic. But maybe it's the characters. Everyone has their favourite, or Grandpa Joe will remind them of their dad or something, so it's like a big cast of characters where there's something for everybody. A bit like the Spice Girls.

Are you surprised that the younger generation have taken to it as well?

It doesn't totally surprise me because it's a show about being young. A lot of the fans are super young. I've seen a Derry Girls birthday cake for a four-year-old, which is obviously too young to actually watch and understand it, but they must have seen pictures of the characters and they look bright and fun, so they like it.

Have you enjoyed watching the cast go on to do other projects in between filming?

It's been incredible to watch because it feels like yesterday that we began this journey. I remember the first auditions so clearly, and now they're on the cover of all these magazines and they're leading shows. It makes me very proud.

"It's such a privilege and honour"

What's next for you?

I'm doing another thriller with Toby [husband Tobias Beer]. He's currently heavy on the script stage of that, and I'll help out when I'm through the other end of Derry Girls. And then I'm going to start writing something else, another comedy, that I've put on hold for a while. It's really exciting but a bit scary because I'm a bit like, 'How do I write other funny people? I don’t even know'. Coming up with a world, how each episode works and all that stuff. But I'm ready.

How have you enjoyed working with Channel 4 on Derry Girls?

This wouldn't have happened anywhere else. It's such a Channel 4 show. The reason it did happen is because of the support I've had. They really, really stand by their comic talent. And even if you falter, they still believe in you, and believe you have a voice. They've been incredible for me, and I know lots of other Northern Irish writers are going, 'Look at Derry Girls' and feeling inspired because we didn't have any sitcoms at the time really. So Channel 4 have been amazing in helping to make this happen.

Derry Girls - Tuesday, Channel 4, 9:15pm

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