In the 26th Century, the prolonged war between humanity and the fanatical alien alliance the Covenant has ended with a tenuous treaty. Despite the ceasefire, Earth's outer colonies remain vulnerable to the Covenant's covert intrusions. The ONI - Office of Naval Intelligence has been tasked with counterintelligence to beat the Covenant. In Planet Sedra, Commander Jameson Locke and his team witness a Covenant's spacecraft and a Zealot Elite warrior disembarks with a bomb. They unsuccessfully try to stop the alien that explodes the bomb in a mall. They realize that it is a biological attack with an element fatal to humans. Sedran Commander Aiken captures the middleman Axl that tells that the element is obtained from Alpha Halo, where the day longs 16 h and the temperature in the sunlight reaches 482o C. Aiken, Locke and his team head to Halo in the spacecraft Condor with pilot Macer carrying a Havoc weapon to destroy the place. On the arrival, they find two terrorists but are trapped in Halo by weird worms that are attracted by technology. And in five hours the sun will rise.