During the second mission, behind the black market shop you can meet Mindy Blanchard. She'll offer to help you get to the island if you retriever a body for her being held in the Overseer's offices. The body is on the fourth floor of the offices (follow the quest marker), in a locked room. The key is on one of the nearby Overseers. After you get the body, head to the marker to deliver it and the achievement will unlock.