Time Lord Peter Capaldi will make a special appearance in new Doctor Who spin-off series Class.
The young adult adventure, written by Patrick Ness and set in the Doctor Who universe, will launch on October 22 when the first two episodes are released on BBC Three.
In a Facebook live-stream with the show's stars, producer Derek Ritchie revealed: "The Doctor is in!"
Class is set in Coal Hill School, which acts like a beacon for monsters across all of space and time.
Charlie (Greg Austin), April (Sophie Hopkins), Ram (Fady Elsayed) and Tanya (Vivian Oparah), alongside their physics teacher Miss Quill (Katherine Kelly), must fight against the creatures who are struggling to find a way through to Earth.
Peter appears in the opening episode, For Tonight We Might Die, and Sophie admitted to "fangirling" over the Doctor Who actor on set.
"When he was on set the energy was electric, and I think we were all quite blown away and in awe," she said.
"Fangirling! It was like a masterclass watching him. It was like nothing I'd ever seen before. He's so charismatic and he's got his lines in front of him but what you see as the finished product, it's all him. He turns it and makes it his own. It's beautiful to watch."
Her co-star Greg added: "He walked on set and we didn't really know he was there, he kind of just crept on and was like, 'Oh hi guys. I'm Peter, how are you doing?'.
"And watching him take the script from the page and transform it into something real is just beautiful to watch. He's a madman, I think, just in the best possible way."
Peter, who plays the 12th Doctor, said in a statement: "The Doctor Who family is growing, and it's fantastic to be able to welcome the young new cast of Class in to the Whoniverse."
The cast also warned that fans should expect the young adult story to be "very dark" and not suitable for young children.
Sophie explained: "I think people expected it to be for a much younger audience than it is.
"Here's an example: on the first day of the special effects and prosthetics, there was so much blood we felt sick. It is something the Doctor Who universe maybe hasn't seen before in regards to how raw it is, and gritty."
Derek said fans could expect a "brand-new take on the Doctor Who universe".
"We have awesome monsters, we have big concepts, we go to wonderful places," he said. "But at its heart, it is the story of our gang, and we follow that story so closely.
"Every aspect of their lives we investigate and that makes it really unique, because we see these people in their real lives dealing with the most insane things every single week and still trying to go to the prom and still trying to have a relationship and still trying to pass their A-Levels while the most bonkers things happen to them that change their lives forever.
"So it's incredibly heartfelt, it's incredibly urgent, it's incredibly real. I think it will feel like a really unique but very grounded part of the Doctor Who universe."