Why Every Classic Companion Left Doctor Who
25. Jean Marsh - Sara Kingdom
Sara Kingdom teamed up with the First Doctor and Steven to foil the Dalek's Masterplan, before being killed by their Time Destructor. Jean Marsh never saw Sara Kingdom - the character she played in The Dalek's Masterplan - as an official Doctor Who companion, and said as much in DVD special features.
However, because of all her TARDIS travel during the epic adventure that is The Dalek's Masterplan, Sara has been given an honorary status as companion.
Beyond a planned appearance in Terry Nation's potential Dalek spinoff, there wasn't a serious intention to keep Sara beyond Masterplan. And so, Marsh left because her (substantial) guest role in DMP had come to an end. Marsh would later return alongside Nicholas Courtney (who played her half brother Bret Vyon in Masterplan) in 1989's Battlefield.