Here is an overview of Campaign, via Microsoft:
When all hope is lost and humanity’s fate hangs in the balance, the Master Chief is ready to confront the most ruthless foe he’s ever faced. Step inside the armor of humanity’s greatest hero to experience an epic adventure and explore the vast reaches of Zeta Halo, from stunning heights to mysterious depths below the ring. Rescue UNSC marines to gain reinforcements in your fight against a fearsome enemy known as “the Banished.” Enjoy true Spartan freedom in the biggest, most wide open and adventure-filled Halo experience yet.
Watch the trailer below.