Our flight through Season 4 continues, plunging underwater and crash landing on the moon with The Highlanders, The Underwater Menace and The Moonbase. Nothing in ze vorld can stop us now!
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Things get even more complicated with The Underwater Menace. Episode 3 was included in the Lost in Time box set. (Amazon US) (Amazon UK). Episode 2 was rediscovered in 2011, and is the only extant episode of the entire show not yet released on DVD . An audio version exists, narrated by the charming Anneke Wills. (Audible US) (Audible UK)
Oh my God. Here's Hannah Gordon singing The Windmills of Your Mind on Morecambe & Wise in 1973.
The Underwater Menace
Here is a lovely picture of some Fish People.
And here, from the BBC website, is the scene between Troughton and Professor Zaroff from episode 2.
David Banks wrote an exhausting history of the Cybermen which was published in the 1990s. (Amazon UK). So there's that then.
The delightful Damian Shanahan is responsible for finding many surviving clips from otherwise lost episodes, clips that were cut from the program by the ABC's censors because they were too violent. You can read about this on Steve Phillips's Doctor Who Clips List website, where the surviving clips are exhaustively catalogued.