Two former Doctor Who companions will appear in Jodie Whittaker's final adventure later this year.
Tegan and Ace will join the 13th Doctor in the special episode being broadcast this autumn.
The news was revealed in a trailer that followed the show's Easter special.
The short teaser also revealed that the episode will feature the Doctor's arch enemy The Master (Sacha Dhawan) and two of her most famous foes, the Daleks and the Cybermen.
Tegan Jovanka, an Australian air stewardess, appeared on the BBC One show between 1981 and 1984 as a companion to Tom Baker and Peter Davison's Doctors.
It's the first time she's been seen on screen in almost 40 years.
She's played by Janet Fielding, who says: "In some ways it was a very different experience to what it was like when I finished recording in 1983, but in many ways it was very similar. It was so lovely to be a working member of the Doctor Who family again."
Ace was in the TV drama from 1987 to 1989 alongside Sylvester McCoy's seventh Doctor.
Actress Sophie Aldred says: "It's been quite a challenge to have such a big secret to keep, even from my family, and I couldn't be more thrilled and excited to have been asked back. I hope everyone enjoys it as much as I adored being part of the TARDIS team again."
She was the show's final companion before it went on a 16-year hiatus. Writer/producer Russell T Davies revived it in 2005 with Christopher Eccleston playing the returning Doctor.
Whittaker, the first woman to play the Doctor, has appeared in three series of the sci-fi drama. Her first series in 2018 was one of its most-watched ever.
She announced that she was quitting the role last summer, and that she would be saying goodbye to the character in three specials.
Her replacement as the Doctor is expected to be revealed in the coming weeks.