I guess a good way to start off is to figure out a main character to have the game be staged around. I'll start off by describing this character and maybe giving him a background just to start off.

The year, 3557 A.D.

Earth has long researched the methods of light speed and travel and has stumbled upon many new worlds. One of these worlds was of an advanced race that called them the Trayng. Here on Trayn, they are in a peaceful state do to the fact that only police are permitted to carry projectile weapons. After discovering this, the ruler of all of Earth, Emperor Sidar Faylin decides that if enslaved, the Trayng could be great slaves in engineering weapons to use on the rebel factions on Earth. He decides to use forces instead of negotiations because the peaceful dwellers on Trayn would never make weapons for his use willingly.

While explaining this to his advisor, Sidar fails to notice that Cain, head intelligence operative of Trayn, was in the room well hidden listening to every word of it. Cain rushes to the ruler of Trayn to tell him what he's heard but king decides that if he makes a deal with Sidar they can avoid a war. Cain decides to take matters into his own hands. He quickly seeks out his eldest son and his friends and tells them the whole story and that they could be the only hope for they people.

For about 2 years (roughly the time it took for Sidar to go back to Earth and come back with a sizable force), Cain concentrates on training his son and his friends to be deadly hunters and assassins. He was becoming proud of each and every one of them and decided in about another year, their training would be complete and they would be able to tackle anything. Unfortunately, Earth's forces struck the very next day...

When making his daily rounds in the castle's Starport, Cain saw something that filled him with more despair then his worst fears... Earth's warships broke through the planetary shield... with one lucky shot, Cain became the first victim in an all out war, futilessly fought by the Trayng. In the end, Earth's forces broke through and captured the king of Trayn. With him as a hostage, the remaining Trayng surrendered and most were shipped off to secret space colonies to begin work on new war technologies.

When taking in roll to make sure all had been taken, Sidar noticed that 5(or however main characters u want) individuals were neither captured or found dead. The only question remains, will Cain's students' training be enough, or is their struggle as futile as their teachers?