Call The Midwife star Jessica Raine will play the woman who brought Doctor Who to television in a one-off show marking the 50th anniversary of the sci-fi series.
The actress, who plays nurse Jenny Lee in the hit show, has signed up to play television producer Verity Lambert in the 90-minute BBC2 drama An Adventure In Space And Time.
She is joined by Harry Potter star David Bradley, who will play the first doctor - William Hartnell.
David, who said he took the role after being approached by the show's writer Mark Gatiss, said he was "absolutely thrilled".
He said: "I first heard about this role from Mark while watching the Diamond Jubilee flotilla from the roof of the National Theatre. When he asked if I would be interested, I almost bit his hand off.
"Mark has written such a wonderful script, not only about the birth of a cultural phenomenon, but a moment in television's history.
"William Hartnell was one of the finest character actors of our time and as a fan I want to make sure that I do him justice. I'm so looking forward to getting started."
Bourne Supremacy actor Brian Cox has been cast as BBC head of drama Sydney Newman, who is credited with inventing the show, and Waris Hussein, who directed the first episode - An Unearthly Child - will be played by Sacha Dhawan.
Mark said: "What a cast. I'm utterly delighted that everyone's favourite Time Lord will be in such brilliant and stellar company. We have a terrific team who can't wait to tell the fascinating and surprising story of how the Doctor began his journey through space and time."
Filming starts next month.