The game takes place in the year 2552. The Covenant have invaded Earth. Masterchief is missing - he teleported with a covenant cruiser. You the last hope of humanity - you have to destroy all the five Covenant cruisers and protect everyone on Earth.
Complete the game on all difficulties to unlock Mythic mode. Finally, complete the game on Mythic mode to unlock the Plasma Sword. To use the Plasma Sword, kill an Elite that carries one.