Complex sentence: Although a few earlier stories contain passing references to the Doctor's unseen exploits (for example, "The Celestial Toymaker"), this is the first story where the Doctor's previous knowledge of the setting and its characters are crucial to the story's development.
Split into simpler sentences: Although a few earlier stories contain passing references to the Doctor's unseen exploits (for example, "The Celestial Toymaker"), this is the first story to be constructed around the concept. The Doctor's previous knowledge of the setting and its characters are crucial to the story's development.