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

This is a story related achievement and is earned after you meet Doctor in Strelok's hideout in the Agroprom Underground. This quest shows up after you speak with Guide in Cordon. Do note that rarely Guide can die and this quest won't pop up, making you have to reload your save or possibly the game if you haven't been saving often. The Agroprom Underground will not have changed since the first visit other than the possibility of some bandits and bloodsuckers spawning. Once you start climbing the ladder into the hideout a cutscene will play out and the achievement will unlock.

Note: This quest is part of the questline for the "good" ending which starts with talking to Guide. All of these quests are optional and are not required to beat the game, but many story achievements will not be unlocked if this path is ignored and there is no other way to unlock the "good" ending.