Complex sentence: The pilot film was initially shown in cinemas in the spring, and good box-office returns led NBC to commission a full series, which started in September 1979 with a modified version of the pilot film.
Split into simpler sentences: The pilot film was initially shown in cinemas in spring 1979. Good box-office returns led NBC to commission a full series, which started in September 1979 with a modified version of the pilot film.