In the episode titled "Planet of the Dead," Ryan, 24, will portray the mysterious Lady Christina de Souza.
"As always the script is being kept strictly under wraps," says executive producer and writer Russell T. Davies. "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."
Joining in on the fun is Evans, who will play Malcolm, whose life becomes somehow connected to the Time Lord's through unusual circumstances.
Filming on "Planet of the Dead" has already begun in Wales. It is the first of four "Doctor Who" specials, marking the end of David Tennant's run as The Doctor before Matt Smith takes over for the 2010 season.
Ryan's participation in the series has caused speculation that she might be up for the role of the Doctor's assistant. Having already appeared with him in this special shouldn't necessarily preclude her taking the bigger role later. Catherine Tate had appeared in the "Doctor Who" Christmas special with David Tennant first but returned as The Doctor's assistant 15 months later.
Besides starring in the remake of "Bionic Woman," Ryan is known for her roles in "EastEnders" and "Jekyll" in the UK.