Doctor Who: The Five Doctors, on BBC One (UK)
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This first aired in 1983, to celebrate the 20th anniversary of the series Doctor Who. I'd guess I must have first seen it sometime later than that, closer to the end of the 80s, but I'm not really sure. Anyway, it was set between seasons 20 and 21, during the tenure of the fifth Doctor. In addition to him, the second and third Doctors were in the special, played by their original actors, and the first Doctor was also in it, but played by a different actor (since the original was dead), though personally I didn't notice the difference. (Possibly because I wasn't really familiar with the first Doctor, at the time.) The fourth Doctor appeared, but wasn't really involved in the story, because the actor didn't want to participate, I guess. So they used some old footage of him, or something.
Anyway... I remember a bit about the special, though at the time that I watched it I'm not sure I was aware it was a special, per se. I don't remember enough to give a detailed summary, I'll just say that the Master was in it, and it involved Rassilon, an ancient Time Lord who had apparently learned the secret of immortality. It was definitely an interesting story, with some amusing bits. And someday I definitely hope to see it again, and write a proper review.