Chief has gone rogue. Locke is tasked to hunt him down, he needs to learn about Chief and recruits Arbiter to help him. Arbiter then tells of the hero that Chief is, which then leads to the three teaming together to stop the Forerunners.
Agent Locke, the star of Ridley Scott's Halo: Nightfall, is a playable character in Halo 5: Guardians, 343 Industries revealed today during an SDCC panel.