Bennett was bought on as producer of the as-then unnamed Star Trek II in mid-late 1980, primarily because Paramount rated his ability to manage production budgets (the film ended up using a third to a quarter of the budget of the first). He, along with writer Jack Sowards, watched the first two seasons of Star Trek, and selected the first season episode "Space Seed" to write a sequel. Sowards then wrote the screenplay (although most of the screenplay was actually written by the director, Nicholas Meyer).
The film was a huge success, and Bennett was retained as producer for the successful (but poorly received) third film, and the hugely successful fourth. During the making of the fourth however, Bennett's relationship with Leonard Nimoy, the director, and Paramount deteriorated, and Bennett decided to leave, due to the fact that it looked inevitable that Nimoy would direct the fifth film.
Things changed when William Shatner was signed on as director. Bennett came back as producer for the fifth and sixth films after much persuation, but after that, things started to go wrong. ILM, who had done the effects for films II-IV would be unavailable for the best part of 1988 (when the film would have been in production). Using ILM would have required pushing the film for a 1987 release (which would have been near-impossible), or waiting until 1990 (Bennett's preferred option, but Paramount wanted a 1988 or 89 release). Bran Ferren (the FX worker on Little Shop of Horrors) approached Paramount, and said that he could deliver ILM-quality effects at a lower cost. Paramount accepted and slashed $8 million off the budget.
Unfortunately, the results were awful effects that looked about 10-15 years older than they really were. This was a result of Ferren and his team using a combination of outdated methods and newer methods which were not yet mature enough. Moreover, the screenplay ended up being underdeveloped due to a Writer's Guild of America strike, which prevented the screenwriter, David Loughery (as well as Bennett) from being able to do any work on the screenplay. Reshoots meant that the film had to be released in a similar timeframe to Batman, Indiana Jones and the Last Crusade and Ghostbusters 2, and despite a strong opening weekend, the film's box-office collapsed and ended up making barely half that of the previous entry. Reviews of the film were probably the worst out of any major movie that year, and it has since generally become regarded as Paramount's worst-ever major release.
Bennett believed the problem was that the crew had become stagnant, and pitched an idea which would show the early days of the crew at Starfleet Academy. However, Paramount chose to follow an idea by Leonard Nimoy and Nicholas Meyer, and removed Bennett from the series. Since then, Bennett has never worked in any major capacity in the film or TV industry.