Complex sentence: Actress Elizabeth Shepherd was originally cast as Emma Peel, one complete episode "The Town of No Return" was filmed before the producers closed down production partway through filming of the second episode "The Murder Market" in order to recast the part.
Split into simpler sentences: Actress Elizabeth Shepherd was originally cast as Emma Peel; one complete episode, "The Town of No Return," was filmed. Partway through filming of the second episode, "The Murder Market," the producers closed down production in order to recast the part.