Right, I'll let you guys fill in the blanks for Chapter 3.
We escape the Dark Eldar and rendezvous with Inquisitor Conway on the moonbase at Sheol XVII. From there we theorise that the missing sections of the Acherade may have actually completed the scheduled warp jump to Sigurd IV.
We therefore travel to Sigurd IV, a hive world where the missing sections have crashed. There we meet the Inquisitor's contact on that planet.
*Points to Orion*
But, just to complicate things, the hive world is also currently under attack by ______
And the _____________ Chapter of Space Marines have been called in to help.