Shada

Shada: a prison built by the Time Lords for defeated would-be conquerors of the universe. A scientist named Skagra needs the help of one of the prison's inmates. He finds nobody knows where Shada is anymore, except one aged Time Lord who has retired to Earth, where he is a professor at St Cedd's College. Luckily for the universe, Skagra's attempt to force the information out of Professor Chronotis coincides with a visit by the professor's old friend, the Fourth Doctor.

Note: This story never actually aired on TV in 1980, although it was planned to. A combination of rampant inflation in Britain and union strikes halted production partway through filming.

The story finally was aired on television for the first time on 18 July 2018, on BBC America. This marked thirty-eight years, nine months and three days between the beginning of filming and the airdate, a record for any episode of Doctor Who.