Years and years ago (or in the future), humans colonized space. The planet on which the story takes place, was left to fend for itself long ago.  The planet has very limited resources, and while the humans who have survived have records of the science and achievements belonging to their kind across the universe, they see it all as having little relevance to them and their daily lives. There are a few primitive alien lifeforms that inhabit this planet, but they are assumed to be the minority.

The aliens that are observed by the humans are crazy monsters. In fact, there's a whole civilization of beings (overworld, underworld, or whatever). The mindless raaawwwrrs the humans deal with are essentially pawns/scouts. Humans have a repository of knowledge, but they don't really give a shit about it.  Most people do know about it, though, and it's a place that our hero has always wanted to visit, but has never had the chance.

Several years ago, the whereabouts of this repository were made known to one of the alien species, they found it, copied shit, and realized what could be made possible if some kind of fuel were to be discovered on the planet. These aliens did find a source of fuel, but it's completely inaccessible and/or unusable without some kind of key.  They realize the key lies within a device kept secret for years, that is currently in our hero's wife’s uncle's possession.  The uncle raised his niece (hero's wife) from birth.

Both the uncle and hero are part of their small settlements military force (militia, or whatever).
The hero is off doing his thing, he comes back home, and his home is thrashed, and his wife is gone.  He has no idea why anyone would want to take his wife.  He goes to the uncle, and while the uncle knows what they want, he doesn't let it on, but he does say he knows who took his wife, though he can't say whether she is dead or alive.  They embark on the journey to get to the fuckers, and a short while in, the uncle has to make use of his secret device. He never thought he'd have to.  Our hero WTF's, because it's a cannon/rifle/thing (futuristic distance weapon meant to be used for special attacks and has its own charge, replacing the more common MP skills seen in these types of games).  At a boss, the uncle "dies", hero gets the weapon.

But of course uncle will have over done it in that fight with the badass cannon so it gets nigged down to a piss, giving the player the ability (responsibility) to power it back up.  As far as upgrading it, let that be the sole doing of the hero, since he's all knowledgeable about science and shit from his huge interest in it.

The latter parts of the story would be the humans on the planet wanting to use the power to go to war with the rest of the humans to get back at them for abandoning them, and the main characters being like, "What, no way!"  A struggle ensuing as a result.