Why Every Classic Companion Left Doctor Who
16. Caroline John - Liz Shaw
Liz Shaw was UNIT's first official scientific advisor, but was quickly demoted to lab assistant when the Third Doctor turned up on the scene.
This was one of the reasons that Caroline John grew disillusioned with the role of Liz Shaw. This, and falling pregnant with her first child meant that John was strongly leaving Doctor Who as season 7 came to an end. However, the decision was ultimately taken out of her hands.
Producer Barry Letts felt that Liz Shaw was too over-qualified to be a mere assistant to the Doctor, and so the character was hastily written out between seasons 7 and 8.