Star Trek: Tactical Assault takes place during the original crew time period. You are a new captain given your first command over a Star Fleet vessel and after a couple of training missions, you are off to help protect the Federation-Klingon Neutral Zone.
Missions typically follow a pattern of getting a hail from Star Fleet, warping to a destination, hailing the people you are there to help, hailing the antagonist and more times than not, duking it out with the enemy. Eventually, you will also be able to unlock a storyline following the Klingon Empire, but the overall feel of the game doesn't really change much and this same pattern persists in the other story.
While in combat (after raising your alert status to red), you will be able to see six color-coded arcs around you and your enemy's ships. These arcs act as your shields. They start off green, but as they take damage, they turn to yellow and orange and red before they go out, thus leaving an opening in the ship's defenses. Also while in battle, you can scan your opponents and get a run-down on their weapons and various statuses to help you with your strategy.
One of the more interesting aspects of this game is that each mission (theoretically) has multiple outcomes. For instance, just after your initial training, you receive a distress call from a cargo ship that is being attacked by Andorians. Just when you have the enemy at your mercy, you are called back to the station. What happens if you follow orders? What happens if you finish the job first? There are also branching dialogue trees that occur during or before most missions. There might be a way to avoid the fight altogether if you choose the right path, but then again, most of the missions will lead to an inevitable battle.
Throughout the game, you will encounter three types of ships (either piloted by you or by your enemy). There are, of course, the Federation ships, but also the Klingon and Romulan Birds of Prey to deal with. Each fleet's ships are designed differently (not only visually, but functionally as well). Where Federation ships use phasers and photon torpedoes, Klingons share the torpedoes, but have disruptors instead of phasers (the difference being recharge time and destructive capabilities). And the Romulan vessels use disruptors and plasma bolts (very inaccurate, but very powerful beam weapon).