The campaign setting, if you were to use the Valiance & Co. in roleplay, is essentially X-COM in space. The main fleet of humanity is out fighting a war on a distant front, and doing average to well, when a second mysterious force shows up, glassing an outskirts planet way over on the opposite end of explored space. The only ships available are outdated mothballs from the "bigger is better" era (The Valiance), mostly decorative parade ships (The Testament), old battleships retrofitted into armored transports (The Guardian), and half-finished exploration/science vessels (The Galileo). These ships are sent out because all the high-ups think the current enemy is the real threat, and this new mystery foe is nothing major (BIG MISTAKE).
The captains are the PCs, and are all inexperienced officers on their first commands. The crew is a bunch of dammed X-COM Rookies, their assigned resources are abysmal, and the enemy is totally unknown. The PCs have to turn their force into something capable of holding off the new foe. They'll need to fend off attacks by unknown ships with new and frightening capabilities, research the debris of said ships to create countermeasures and learn of the alien technologies, protect various colonies and outposts, and expand their forces with more ships either salvaged from ancient ship scrapyards, converted from civilian use, or built with whatever they can scrounge up.