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.”