I'm new at this, but i have been reading the halo novels lately and have come up with a few probable story lines for halo reach.
1. the first and most likely is the story of the fall of reach the main industrial planet and base of operations for the UNSC. Before the master chief took off on his mission to capture a profit his ship receives a distress call from reach telling them of a immanent attack. the ship return to reach to fight alongside hundreds of other unsc ships. When the covenant go to ground to destroy the reactors of the super mac guns the majority of the spartans go to ground to defend the reactors, while master chief boards an abandoned unsc ship to destroy its database before the covenant find it and locate earth. It is when the spartans defend the reactor is when i think the most likely scenario for the game starts. fred (a temporarily in command spartan) leads the team into defensive positions around the reactors, many spartans are lost and the reactors fall to the covenant. the team is split in half, one side to go help a high ranking officer setup a planetary destroyer nuke with a timer set for ten days, while the rest set up camp in an underground base known as castle base where the spartans where trained. Then when the base is invaded they are sent even further underground where they find tunnels setup by the forerunners and a chamber where they find a crystal so desperately wanted by the covenant they wont try and kill them. It is at this point that the master chief returns, in a stolen covenant ship to save the remaining spartans. when they make a jump to slip-space they find themselves trapped outside of time and space... and if you want to hear the rest of the story read the books because it gets a bit irrelevant after that. The spartans in the picture, the only screenshot released by Bungie show the spartans and their different characteristics. Fred in the middle, kelly with her sniper on the right and I'm guessing grace in between, the unusually oversized sam on the left and in between someone I don't think is a spartan, catherine Haynes lead scientist on the spartan 2 program.
2. The second most likely scenario is that chief Mendez who trained the spartans and mentioned training a second batch has succeeded and they are now fighting to save reach. If that is the case than these spartans will not have as updated mjlinir armor because the armor master chief has he only just received before his latest mission.
[Edited on 07.29.2009 5:01 PM PDT]