Star Trek, Behind The Scenes

The threat of cancellation loomed during the show's second season. The show's fan base conducted an unprecedented letter-writing campaign, petitioning NBC to keep the show on the air. NBC renewed the show, but moved it from primetime to the "Friday night death slot", and substantially reduced its budget. Roddenberry reduced his direct involvement in Star Trek before the start of the season to protest the changed timeslot, and was replaced by Fred Freiberger.
The series was canceled in its third season, despite the protests of a renewed letter-writing campaign. Marketing personnel of the network complained to management that the series' cancellation was premature. New techniques for profiling demographics of the viewing audience later showed that Star Trek had been highly profitable for advertisers, though this news came too late to resume production of the series.