S.T.A.L.K.E.R.: Shadow of Chornobyl

This is a story related achievement and is unlocked after you talk to Guide in Cordon. After you grab the documents in Lab X-16 for the ecologists this quest will pop up, however, it is not necessary to do this quest and all other subsequent quests associated with it to beat the game, after X-16 you can continue the path of quests to go to the CNPP and beat the game. If you want to get all of the story achievements I would recommend going to Cordon to talk to Guide and finish this quest to get started on the path to the "good" ending. Do note that while it is rare, it is entirely possible for Guide to be killed by dogs before you can reach him and lock you out of this achievement. If this happens reload the autosave and hope you can reach him before the dogs do.