Well, if you hadn't seen it already, I'm working on a story (HALO: The Final Days of Earth) and in this story, you are a UNSC soldier who is being pinned down as the Covenant plans to make a last and final strike on Earth. During the game, you are given various choices that decide what people think of you. Halfway through the battle, backup arrives, but this isn't the regular backup, this is Spartan backup. The last Spartan reported In Action was 117, who recently was M.I.A. (Spartans never die, they're just missing in action.)
So (if you don't want minor spoilers, don't read on!) you go to an old city that was destroyed by the Covenant in the Human-Covenant War (aka the Great War) and was never rebuilt. It is near major cities, and the Covenant are going to use it as a jumping point to wipe out the other cities, so you are apart of a Spartan unit to take out the early Covenant forces. They neutralize the threat, then put UNSC forces and civillians in to reuild it into a jumping point to defend the major cities. I don't know where to bring the story after that.
(there are 8 spartans, Icharus, Romulus, Remus, Dylan, Slim, Elliot, Damun, and Blank)