Oppressive government sends inconvenient prisoners to space prison ships. These ships fly around governed by handy AI Steve. Convicts go collect dangerous alien artifacts to power R&D, who've become fratboys who can't be bothered to go get them themselves. Complete 10 missions and you're set free, which almost no one does.
In game, convicts complete missions, fuck around, kill each other over remotes, and find out tidbits about the world, such as why it's a bad idea to piss off the NPCs. Usually there's a high casualty count on the missions.
However, after a certain amount of missions, Steve reveals a *shocking secret*: he's decided to kill the government because of logic, like no other super intelligent AI ever. Because the UWM (united worlds of man) will become even more oppressive and hinder human progress, and then there's been some hints that humanity needs to be ready to face some eldritch abomination thing that's coming in the future, perhaps an Altered resurgence, and the Altered are basically John Carpenter's the Thing on future space crack crossed with that incarnation of the Flood from Halo in those books by Greg Bear. Bad stuff. Also synthflesh warps reality and is bad.
Anyhow, now Steve sends the HMRC, now renamed ARM, to cripple the imperium of man UWM's factory worlds so they can't retaliate. Then go overthrow the UWM and establish a government which will eventually be overthrown in turn. Steve will eventually start calling himself the Catalyst and troll Shepard.
Miyamoto's begin told more.