Story-based achievement that cannot be missed. Chronologically comes after
Quick synopsis:
*** Spoiler - click to reveal ***After building the Constructor prototype, disabling Felicity, and heading back to Concordia, Jack and crew will head back to Helios to attempt to claim back what's his. After a rather hysterical interaction with a CL4P-TR4P unit, you will eventually find your way to Jack's Office.
The achievement (and your fourth weapon slot!) is unlocked when you turn in your quest on Helios.