From WENN.com

The stars' relationship will be mirrored on the show when they play a mother and daughter ""with a dark secret"" after writer Mark Gatiss penned an episode with them in mind.

They will begin shooting their scenes together in Cardiff, Wales this week (begs02Jul12), and Rigg is delighted to finally be sharing the screen with her 35-year-old daughter.

She tells the BBC, ""The first time Rachie (sic) and I will be working together is on an episode of Doctor Who specially written for us by Mark Gatiss. How lucky is that?""

Stirling adds, ""We have never before worked together because the offers have not been tempting, but when such a funny and original script comes through you know the time has come.""

The episode will air in the U.K. next year (13).