 When the series began, Ian Hendry (I) (qv) was the main star, with  the idea being he would rotate between different partners (an early  version of the _"Mission: Impossible" (1966)_ (qv) format). The  series title actually refers to Hendrys character, Dr. David Keel,  and Steed, who worked together to find those responsible for Keels  wifes murder in the first episode. Early episodes focused more on  Keels character, and Steed doesnt even appear in a couple! When  the first season was interrupted by a strike, Ian Hendry (I) (qv)  quit the series during the hiatus. The same format was used for  Steed with a couple of leftover Dr. Keel scripts retooled for a new  character named Dr. Martin King, but other scripts, originally  written for a male character, were rejigged for another new  addition: Cathy Gale.