 On the ringworld , the Chief rallies human survivors and leads a boarding party to rescue Keyes from the Covenant 's clutches . Keyes reveals that the Covenant call the ring " Halo " , and they believe it is some sort of weapon . The Chief is tasked with finding Halo 's control room before the Covenant does . Once Cortana is inserted into the control room , she becomes alarmed and stays behind while she sends Master Chief to find Keyes . While searching for the captain , Master Chief encounters the Flood , a parasitic organism that infects sentient life . The release prompts Halo 's caretaker , the AI 343 Guilty Spark , to enlist the Chief 's help in activating Halo 's defenses . The Chief 's activation of the ring from the Control Room is stopped by Cortana , who reveals that Halo 's defenses do not kill Flood , but rather their food in an effort to starve them — meaning that activating the ring would wipe out all sentient life in the galaxy . To stop the Flood from spreading and Spark from activating the ring , Cortana devises a plan to detonate the crashed Autumn 's engines and destroy Halo . Fighting through Flood , Covenant , and Guilty Spark 's robotic Sentinels , the Chief manually destabilizes the Autumn 's reactors and he and Cortana narrowly escape the destruction of the ring via a fighter . 
