The Three Doctors (Doctor Who) the Third Doctor (Jon Pertwee) for help when Omega drains their civilisation's power. The serial opened the tenth anniversary year of the series, and features the first three Doctors all appearing in the same serial. This makes it the first "Doctor Who" story in which an earlier incarnation of the Doctor returns to the show. A superluminal signal is sent to Earth, carrying with it an energy blob that seems intent on capturing the Doctor. The homeworld of the Time Lords is also under siege, their power being drained through a black hole. Trapped themselves, and desperate to send help,
Enlightenment (Doctor Who) serial "Mawdryn Undead", and concluding with "Enlightenment"), the enemy was the Black Guardian, who was last encountered by the fourth incarnation of the Doctor at the conclusion of The Key to Time saga in the 1979 serial "The Armageddon Factor". The story also saw the return of the White Guardian, who had also not been seen since 1979. After penning a number of radio and TV scripts, including episodes of "Crossroads" and "Waggoner's Walk", Barbara Clegg submitted a story idea to "Doctor Who" script editor Eric Saward, an acquaintance from the BBC drama department. Interested in writing for the series,