Tracy-Ann first appeared as Yvonne in the 2006 Doctor Who episode Army of Ghosts written by Torchwood creator Russell T Davies. The ruthless head of London's Torchwood One, her universe-threatening schemes placed her at odds with the Tenth Doctor (David Tennant).

Yvonne perished in a city wide battle, but this new release takes place in Cardiff a year before her death on a secret mission involving a mysterious alien conspiracy. She's brought her dress sense and her marvellous people skills... and she's no idea what she's in for.

'When we presented the idea of using Yvonne,' says producer James Goss, '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: One Rule has been written by prolific author Joseph Lidster – whose previous credits include the second series Torchwood episode A Day in the Death.

'Torchwood was my first piece of writing for television and it remains my favourite series.' says Joseph, '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.'

Torchwood: One Rule will be released in December from the Big Finish website.

The first title, Torchwood: The Conspiracy starring John Barrowman, is available buy today, exclusive to the Big Finish website. The series will continue with Torchwood: Fall to Earth in October, starring Gareth David-Lloyd as Ianto Jones.

Series 1 and Series 2 are both available to subscribe to today, with Series 1 keeping its special introductory price of £45 on CD and £40 to download until the end of September.