The story mode continues the story started in . There are nine levels in the story, split between controlling Master Chief and the alien Arbiter. There are four difficulties to the campaign "easy", "standard", "heroic" and "legendary" Completing them unlocks extras such as different dialogue in certain sections of the game, and extra weapons in split-screen multiplayer. There are various 'skulls' at hidden parts of the levels, as in Halo 3. These skulls unlock helpful and or strange differences in the levels such as confetti and balloons appearing when the player scores a headshot. The story can be played two player on the same console but not over Xbox Live. 