If I were to summarize: a lot like Lost, but with a triple dose of Lovecraft. Delightfully unsettling. 
 Area X is a patch of pristine wilderness which threatens to incomprehensibly reshape all humans who pass through. It's an uncanny kind of nature that seems to probe at the boundaries of the observer. In the hope of studying it and stopping its possible spread, the government sends in expedition after doomed expedition, arming team members with half-truths and outdated weaponry. As the narrator ("the biologist") says mid-expedition: "Transformations were taking place here, and as much as I had felt part of a "natural" landscape on my trek to the lighthouse, I could not deny that these habitats were transitional in a deeply unnatural way." 
 The narrator is terse, manipulative, untrusting, with greater affinity for nature than with the subtleties of human interaction-- traits which seem to prime her for a quite perverse kind of personal transformation.