In this fast-paced instance, you and your party are trapped aboard the Republic cruiser called the Esseles. Like Darth Vader asking for the ambassador on board the Corellian Corvette at the beginning of A New Hope, the commander of an Imperial Star Destroyer in this Flashpoint is also seeking a Republic ambassador. The difference here is the Imperial forces haven't boarded yet. It is your job to hold them off.
After the break, catch the full 13-and-a-half-minute walkthrough narrated by Director of Production Dallas Dickinson, sans epic beard unfortunately.