The twentieth season of Doctor Who began on 3 January 1983 with the story Arc of Infinity, and ended 16 March 1983 with The King's Demons. A 20th Anniversary special, The Five Doctors, followed in November 1983.
To commemorate the twentieth season, the stories in this season involve the return of previous villains.
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Hmm. I wonder whether the rest of series 7 of New Who might do the same for the 50th.
We know there will be a 50th anniversary special on 23 November 2013, and we have:
New look Ice Warriors
New look Cybermen:
and an Egyptian evil....
by Neil Cross
The Pyramids of Mars