3750. People have long known that three million years ago three intelligent races lived in space, which were believed to have been swept away by the destructive migration of the Forerunners. And now the sudden discovery by the mapping cruiser "Terra" of a lost colony from the era of the Great Exodus refutes the claims of historians. Part of the Insect civilization has survived and now their degraded descendants are waging a mutually destructive war with the people who have colonized their world for the second time.