Ryan, who is best known for her roles as Zoe Slater in EastEnders, and Jaime Sommers in the recent remake of Bionic Woman, will play the mysterious Lady Christina de Souza who joins David Tennant, as the Doctor, on a bus trip which takes a very unexpected detour into danger. She said "I'm a huge fan of Doctor Who and very excited to be joining David Tennant and the Doctor Who team. It is such a fantastic show and I can’t wait to get started!"
One of Britain's best loved comedy stars, Lee Evans, will also be joining the cast playing a character called Malcolm, whose life becomes connected to the Doctor's under extraordinary circumstances.
Noma Dumezweni returns as Captain Erisa Magambo, last seen helping Rose and Donna save the world in Turn Left.
Executive Producer and writer Russell T Davies said "Michelle is one of the most sought after young actors in the country and we are delighted to announce that she will be joining the team. As always the script is being kept strictly under wraps however we can reveal that Lady Christina is a woman with a mysterious past who's going to have a huge impact on the Doctor! "
The episode will be screened at Easter.