In the chapter 'Princess and Tiara', you will be in the barn loft with a bunch of puzzles to solve in order to open a box. Make sure you solve the puzzles instead of breaking open the box.
The first puzzle is located on the back wall and requires you to press the following buttons: Hand, Tree, Princess, Castle, Title
The second puzzle is on the left side wall and requires you to slide the pieces to these positions (from top to bottom): 1 left (Dove), 2 left (Ballerina), 2 right (Engineering), 1 right (Painter)
The final puzzle is just behind you and requires you to push the buttons in this order: Castle, Flowers, Clock, House, Dinner Party
You will unlock a secret compartment with a story in it, earning your achievement. You can replay any section of the Episode in Isolation mode without overwriting your story progress.