[SmG Battlefire]

Master Chief is going to be the main protagonist while Agent Locke will be the second playable character. So it seems that Master Chief is questioning the UNSC (United Nation Space command) or more specific ONI (Office of Naval Intelligence). So more likely he gone Rogue. So ONI sends their ONI Spartan to find him or maybe even hunt him down.

The huge reason why they are making Halo Master Chief Collection is because they will actually somehow introduce Agent Locke.

My prediction is that there might be a standoff between Locke and Chief. I also think that the Arbiter will have something to do with this standoff. He talked about a great threat that was coming.