Spoiler-free Halo 3 ending information follows – and this will be my last Halo-related post for a while because I’m starting to feel quite the fanboy.
When you finish the campaign mode and watch the ending cinematic and are staring at the “thanks for playing!” screen, do not – for the love of whatever deity you follow – do NOT press “A” to get back to the main menu. After a bit of a pause, there will be a long ending credits scroll.
AFTER the credits, you will see the REST of the end of the story.
If you’ve played through on “Legendary”, there’s even more ending after that, but even on Normal, there’s stuff you’re going to want to see.