The game starts when Dingo, who is piloting a mining mech, stumbles upon a strange package that happens to have the Orbital Frame (what they call combat-ready mechs in this universe) Jehuty inside. As (bad) luck would have it, another group shows up and wants to take it from him almost immediately, destroying his mining mech in the process. Not being one to let another just take what he found, he fights back, and keeps it. Of course, that is far from the end of the story. He pursues those who he believes to be bad actors, and gets captured, which then leads to him getting shot and almost killed, except for the quick work of another who hopes to use his skill in Jehuty to further another goal.
Source: N4G PC Zone of the Enders: The 2nd Runner Mars review – ChristCenteredGamer