Vault of the Cyborgs

 was an episode of Professor X which the Seventh Doctor thought was particularly good when he originally watched it in the 1960s. He changed his opinion when he watched it later in his life. (AUDIO: Klein's Story)

Behind the scenes
With Professor X serving as equivalent to Doctor Who within the Doctor Who universe, Vault of the Cyborgs is a clear analogue to the serial The Tomb of the Cybermen.