Gamespot.com said they think the game trailer is occouring in the future. Personally, my prediction is that the Large flying object the Chief was on at the end of Halo 2 crash landed, which is where the trailer picks up. There is debris everywhere, which could be the effect of a crash landing...
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my guess is that master chief will go back in time in the era of the forerunners. The forerunners are busy controlling the flood and master chief will assist them with containing the danger and he will be the cause of the activation of the rings. the gravekeeper or whats his name will be seen and guilty spark. that would explain their weird behaviour and the confusing things they say. and the quote that cortana made in the trailer i know you, your past your future. she has been told by the gravekeeper. And to back my story up, this all happened in their previous project Marathon and halo is basically a remake.