After destroying the Halo ring and the Flood, Master Chief ends up in the Star Wars universe due to his half of the Forward Unto Dawn being pulled into slip space. There he is found by the Mandalorians during their civil war. After helping the side that saved him and Cortana win, he becomes a mercenary and is eventually recruited to be cloned on Kamino instead of Jango Fett.

After the battle of Chi Ceti IV, the Covenant Ship explodes and creates a Warp rift that drags in the UNSC Commonwealth. The Commonwealth then crashes on an Imperial planet infested with Orks. The Spartans being the only survivors are rescued from certain doom by the Deathwatch and the males, having impressed the Watch Captain are recruited into the Adeptus Astartes.