During Act 4 Part 2, after talking to Dr Amarai, cross the swimming pool to the toilets on the left (the door will not open until after you have talked to the Dr).
After the double doors, through to the right. There are also two items to look at and a conversation with Beth you can do but these are not needed for this achievement. Go through another door on the other side of the room, this will trigger a cut scene. You will get the achievement after the cut scene has finished.