Master Chief is having an epic battle with the Didact. The Didact is about the deliver a final hit to Master Chief that can kill him. The Didact hits Chief and then suddenly the screen flashes to white.
The screen goes back to normal to a kid waking up from a long night's rest. He wakes up and says "Whoa that was an awesome dream!" Then his mom calls him to get ready to go to school.
All of Halo as we know was just a little kid's dream.
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That ending is almost as good as Super Mario Bros. 2's ending!!!