The year is 3053, and the planet Solemn, long forgotten in the wars of the interstellar Great Houses, has had its first visitor in over a decade: the jumpship Silverado and its six Dropships.
However, Solemn has Houses of its own, and the Silverado could prove the tipping point in their own centuries-old Struggles, an endless tug-of-war over territory and resources. There are treaties in place preventing the import of arms, but nobody believes the other two sides will respect those treaties. That makes it a race: who can smuggle enough supplies from the Silverado to decisively win the Struggles, and who will be crushed by a pre-emptive strike using superior numbers and equipment.
Finally, the Silverado brings ten years' worth of news and information to Solemn. Heading the list, and almost beyond belief, is news that invaders from beyond the Periphery have claimed dozens of worlds, some just a few jumps from Solemn. They use technology like none ever seen before in the Inner Sphere. Their mechs are faster, they hit harder, and they're shooting before the Inner Sphere forces can even close range. To top it off, their pilots are genetically engineered and trained from birth to be better mechwarriors than their Inner Sphere counterparts.
The clans have missed Solemn so far, either because it's not strategically important, or it's simply an obscure world with no links to the outside. But nobody thinks that Solemn's luck can hold.