Complex sentence: In 1963, he wrote the Doctor Who story "The Daleks" (aka "The Mutants") for the BBC, which introduced the most popular villians on the show, and was responsible for the BBC's first merchandising boom.
Split into simpler sentences: In 1963, he wrote the "Doctor Who" serial "The Daleks" (aka "The Mutants") for the BBC. The serial introduced the eponymous creatures that would become the show's most popular monsters, and was responsible for the BBC's first merchandising boom.