Welcome to the past! You warp into the system to be told to listen up and hear that the war has not been going well for the Federation and that Klingons are advancing on your position.
Destroy Klingon Frigates that warp in. Once these have been destroyed, repair and re-position if you need to.
Heavier ships will warp in, being Klingon Raiders and Cruisers. During this stage, evacuation ships will launch. You will be told to give them cover. There will be four of these in total, one by one. Look for the lane each time. Each time a lane is cleared, another will appear. When each wave of Klingon ships is defeated, a new wave will arrive. Next up is Klingon Battleships. Still protect any evacuating ships.
Finally, after all four ships have evacuated, destroy any remaining Klingon ships.