John played the role of the Doctor's companion in 1970 opposite Jon Pertwee's Third Doctor. John was recommended to then Doctor Who producer Peter Bryant by another BBC producer, James Cellan Jones, who sent Bryant and his associate Derrick Sherwin photographs of her. Unlike most of the preceding and subsequent female companions of the Doctor, Shaw was a brilliant scientist and understood much of the Doctor's technobabble. Shaw and the Doctor discussed things on a more equitable level of intelligence, and the Doctor respected and rarely patronised her. New series producer Barry Letts believed the character was too intellectual to be a suitable companion to the Doctor and decided against renewing her contract for the next season.
During her final story, "Inferno", John also played the part of Section Leader Elizabeth Shaw, an alter ego of her regular character that the Doctor encounters in an alternative time stream. John reprised the role of Shaw, albeit as a phantom, in the anniversary episode The Five Doctors, and also appeared in the special episode Dimensions in Time (1993), part of the BBC's annual Children in Need appeal. In the 1990s she appeared in a series of straight-to-video releases including The Stranger: Breach of the Peace, and as Liz Shaw in the P.R.O.B.E. stories written by Mark Gatiss and featuring numerous actors from the history of Doctor Who – including Jon Pertwee, Peter Davison, Colin Baker and Sylvester McCoy. In these stories made by the production company BBV, a pipe-smoking Shaw works as an investigator (for the P.R.O.B.E. organisation); John is seen opposite Linda Lusardi in the former model's first acting role.
John more recently appeared in two Big Finish Productions' audio dramas based on Doctor Who; Dust Breeding (2001), although playing a character other than Liz Shaw, and The Blue Tooth (2007) where, as Liz, she recounts in narrative form an adventure she once had with the Doctor and UNIT. After The Blue Tooth she played Liz in four more Companion Chronicle audio plays Binary; The Sentinels of the New Dawn; Shadow of the Past. Her final audio play, The Last Post, which she recorded on 26 January 2012, was released after her death.
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