The Sun reports that new mum Billie, who played Rose Tyler, and Freema Agyeman, who played Martha Jones, will join forces to take on the Doctor's arch rival The Master (John Simm) in the episode which sees newcomer Matt Smith take over as the Doctor.
"This will be the most exciting episode Doctor Who have ever done," an insider said. "Getting Billie to agree is a real coup, but she loved working on the show so much it didn't take much convincing," they continued.