*Spoiler Alert*
Story related and cannot be missed.
*** Spoiler - click to reveal ***After Luke tells you that Nick shot the friend of Walter Luke wants you to hide the knife you found in the small cabin. When searching your backpack you will find out that the knife is gone. Walter is standing outside with the knife. Clementine approuches him and talks to him about his friend. After a while Nick shows up. Whatever choice you made just before that will deicide what happens there. After that the group will appear and the achievement will unlock.