Halo 2 Walkthrough - 1 - The Heretic
The first stage in Halo 2 starts off with a cutscene which shows you the Elite ship commander under tribunal for his failure to protect the Halo. There is mention of The Great Journey, after which the elite is led away under guard.
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You then skip back to the Master Chief, a brief cutscene showing the Chief being grilled for the abuse to his gear by the Gunnery Sergeant. This is the beginning of what you can consider an orientation to the game's controls and backstory. After you hop around abit and do a shield test, Sgt. Johnson will come pick you up and take you on a tram ride while he talks to you about the orbital stations surrounding Earth and how impenetrable the whole setup is.
From here on the cutscene skips back and forth between the Chief being commended for his bravery and the Elite being punished for his failure - an interesting juxtaposition of situations.
The award ceremony is cut short as Covenant capital ships appear outside range of the defense platforms, and boarding ships are sent in to disable the Earth's defenses.
And that's where the actual gameplay begins. You will find yourself weaponless, but if you walk down the stairs to your left you will pick up a Battle Rifle. At the bottom of the stairs there are some SMG's for you to pick up. Hold Y to dual wield these, then following your comrades.