Previous lead actors William Hartnell and Patrick Troughton return to guest as the First and Second Doctors in the programme's tenth anniversary serial, The Three Doctors, although illness limited Hartnell's involvement. This was the first time that previous incarnations of the Doctor had returned to the programme, in what would become a regular feature of anniversary episodes.
At the conclusion of The Three Doctors, the Doctor is finally released from his sentence of exile by the Time Lords, enabling him to travel through time and space once again. This ends the cycle of mostly Earth-bound stories where the Doctor was stranded that began at the conclusion of The War Games in 1969.
The serials Frontier in Space and Planet of the Daleks both feature the Daleks and lead directly from the first into the second, forming a rough twelve-part epic.
When Omega, a pioneer of Time Lord civilisation trapped in another universe, starts ripping our universe apart, the Time Lords enlist the help of the Doctor - all 3 incarnations of him.