Here is an overview of the campaign, via Microsoft:
The Banished have defeated UNSC forces and taken control of the mysterious Zeta Halo, threatening the survival of humankind. When all hope is lost and humanity’s fate hangs in the balance, the Master Chief returns to confront the most ruthless foe he’s ever faced. Here’s a fresh look at the Halo Infinite campaign that introduces players to true Spartan freedom in the biggest, most wide open and adventure-filled Halo experience yet.
Watch the trailer below.