Two decades after the events that nearly brought the Successor States to the brink of all-out war, the Great Houses exist in an uneasy peace. But now, from out beyond the Periphery comes a new threat. A swift-moving military force of unknown origin. Nothing the Inner Sphere has can stop them. Their power, speed, and ferocity are unparalleled. Some of the finest warriors and ablest units have challenged them and been crushed. No force has faced them and won. They are the Clans! A military juggernaut whose sole reason for existence is battle. A race that selectively breeds itself for combat. Humanity's only hope is an alliance of mortal enemies. The Federated Commonwealth and the Draconis Combine, interstellar empires at war for 300 years, must now stand side-by-side or face certain destruction.