Gatiss penned "The Crimson Horror," his second episode of the season (after the Ice Warrior-riffic "Cold War") that airs at 7 p.m. CT May 4 on BBC America. Gatiss was working on a play with Sterling while writing the episode, said "Who" showrunner Steven Moffat, who also works with Gatiss on "Sherlock."
"He said to Rachael, 'Look, I think you and your mom should play the mother and daughter parts in this "Doctor Who" I'm writing,'" Moffat said. "And they were up for it.
"Mark and his little black book--he knows absolutely everybody."
Rigg, who is also starring this season in "Game of Thrones" as the Queen of Thrones, plays Mrs. Gillyflower, who touts the beauty and serenity of her Sweetville mill and the surrounding village, where everyone and everything is perfectly clean and beautiful. But there's something quite odd about the place. Not even Mrs. Gillyflower's daughter, Ada (Sterling), is aware of everything happening in Sweetville and the river nearby.
Says BBC America: "There's something even stranger about the bodies washing up in the river, all bright red and waxy."
Sound scary? It is! And so is Rigg, who knows her way around a juicy part. Off camera, too, she's amazing, said Jenna-Louise Coleman.
"I just love to sit back and watch her," Coleman, who plays Clara, told me earlier. "She's very funny. She's very dry and witty. She makes me laugh. I was just smiling all day when she was around."
"The Crimson Horror" is also special because it reunites The Doctor (Matt Smith) with his Victorian-era pals Madame Vastra (Neve McIntosh), Jenny Flint (Catrin Stewart) and Sontaran commander Strax (Dan Starkey), who acts as their butler/enforcer--not that they can't take care of themselves, as Jenny proves in the episode.
The last time we saw the crime-fighting, interspecies lesbian couple was in "The Snowmen" Christmas special, when Vastra surprised fans with this bit of news about her and Jenny: "Good evening. I'm a lizard woman from the dawn of time. And this is my wife."
The women get a surprise in "The Crimson Horror" when they see Clara, a dead ringer for the Clara who died in "The Snowmen."
Oh that Clara, such a mystery.
Want more? Discuss this article and others on Show Patrol's Facebook page.