Story related can't be missed*
In the Heptadome speak to Lilli to help her enter the mind of Truman Zanotto and discover why he is acting stangely. With Lilli's help progress though Truman's mind and uncover the secret explanation within.
This achievement should unlock after you reunite with Lilli and exit Truman's mind.
* I had to play though twice to get this to unlock. Like others I have had several achievements glitch and not unlock. It seems that using Quick Resume on the series consoles is causing problems with unlocking achievements. Make sure that you quit the game completely each time you finish playing and don't leave this title in Quick Resume. All four of the achievements that didn't unlock for me on the first play through unlocked when not using QR.