Something is destroying ships and stripping planets on the fringe of the Republic. And it is heading for Earth.
Jack should have died at Birth when his family's transport was stranded in a deadly Neutron Star system. Certainly before he reached 6, as his busy Tramp Ship Officer family simply waited for him to (pass on), pretty much ignoring him. His Grandfather University engineering/physics professor, ship schooling him as soon as the could talk. Then at 18, after a few too many drinks to celebrate, he was scooped up by the Republic Navy, that had not seen a war in generations. A Navy desperate for experienced spacemen to man its new Battleships, in a loosing war few people even knew about, yet.
The Ensign, like Jack was the only other surviving child of the neutron star and followed Jack into the Navy to kill him. She was not about to let him get away with what he had done to her even if she died making him pay.
The Admiral serving as a super tanker transport Captain (as short as the Navy was in officers in its rapid build up) while commanding the Tanker Fleet, knew the fleet was on a suicide mission hoping to buy the Republic time to developed ships that could take on the enemy. That is if the fleet could survive long enough to finally find out enough about the enemy to design new ships that could defeat them and transmit that information back to Central Command at The Rock before they died.
Jack realizes that the Battleship fleet he is part of is about to be destroyed after he gets a close look at the enemy. But he now knows what drives their ships and powers their weapons and that with equal or even better ships and weapons, he now knows how to build, they could win the war.
Thus starts a desperate journey to survive incredible odds, long enough to develop or steal the enemies technology. Then test and manufacture the equipment to defeat an enemy that is only known as Spiders, because of the shape of their ships. The only other thing known about them for sure in the beginning, was that they were slaughtering every human ship they came across and enslaving every system in their path, ignoring all attempts to negotiate.