This quest starts on salusa, requires a side trip to wallach, and then finishes back on salusa. Take the salusa ornithoper to the sardaukar base. From there the directions are something like d,s,w,w,s. Explore the area to find out about the plot against the Emperor. You will eventually need an access code to obtain proof of the plot. Illya at the wallach AP has the code. Kill her and she will whisper it to you as she dies. Open the proof, take it back to salusa, and give it to the Boss to complete the quest.
The proof is inside a capsule which is in the trashcan of the kitchen.