If I had no idea what the series entailed, I'd think Gears of War 3 is the video game incarnation of Jack Osbourne's biography; a story of tracking down his long lost father, Ozzy, and shooting at huge monsters, who may or may not be his sister, Kelly.
In all seriousness though, as much as people would believe that Gears of War 3 is online multiplayer only (a la Call of Duty) there is a story you know. You can piece some of it together during this trailer, and for what you're not able to make out, there's, well, actually buying the game. A novel idea!