Tracy-Ann Oberman back as Yvonne Hartman in Torchwood

It’s been three weeks since the Mayor of Cardiff was killed by a shop dummy and the fight is on to see who will replace him.

Yvonne Hartman is visiting the city to retrieve an invaluable alien device. She's in charge of Torchwood One, she's saving the British Empire and she doesn't care about local politics. But she is going to find herself caught up in that fight. There’s a bloodthirsty alien stalking the streets and there’s a special offer on at the all-you-can-eat Chinese buffet.

It’s the 26th of March 2005 and it’s the day that everything changes.

When we presented the idea of using Yvonne, Matt Nicholls at BBC Worldwide came up with a brilliant caveat – he suggested we could have Yvonne, but not the all-powerful Queen Bee of Canary Wharf. Instead he wanted her to come thoroughly unstuck in Cardiff.

Torchwood was my first piece of writing for television and it remains my favourite series. Not just the television episodes but the books, audiobooks, radio plays, all of it. I'd have been gutted not to have been involved in some way - I'd have happily gone into the studio and made the tea!

James Goss gave me a choice of available characters but it was Yvonne who leapt out at me. She's one of my favourite characters to have appeared in Doctor Who - she has questionable beliefs but she genuinely thinks she's doing the best thing for her country. She's also incredibly funny and brilliantly performed by the magnificent Tracy-Ann Oberman.