Prime Directive is a good game to play adventures based on the original Star Trek series. This is not based around The Next Gen or the series that followed. What you get is a interesting and effective system for running games in the Star Fleet Battles universe, which feels more like original Trek. The character gen is based on a points system. You pick your race and which of the six branches you want to be. This gives you your stats, as well as beginning skills. Points are then spent to buy more skills, and increase stats.
The actual game system is fairly unique. Each skill gives the player a number of D6s to roll. The target number is based on what they call a tricode. Made up of three numbers indicating a minimum, moderate or complete success. This system sounds complicated at first, but is actually very easy once you get used to it, and is very effective at building the right feel for the game.
It is a pity the designers did not continue with this version. As the system is good and effective and it helps give you the right feel for the universe. If you want a game that allows you to play in the same sort of universe as James T Kirk and co this is it.