From the introduction:
Spirit possession, having something slide under our skin and take control of our muscles and flesh, dates back to the dawn of time. The idea that through corruption or force any of us could become passive observers in our own bodies is a horror that affects not only our bodies, but our souls. Is it as terrifying for the spirit thrust into an alien environment?
Under The Skin is broken into three sections. This introduction contains a full breakdown of the scenario, complete with backstory, motivations and descriptions of what to expect. Next are scenes, where the story is broken down with scene-specific information, challenges and Storyteller advice on how to make the scenes work for you. Last are the scene cards, meant as a quick resource for you (as the Storyteller) to print and use as place holders and reminders of the principal actions and flow between scenes. As always, the story belongs to you and your troupe, but these cards and other game resources can serve as a touchstone once the characters leave the beaten path.
Under The Skin is designed as a one-shot adventure for Storytellers to introduce new or experienced players to Hunter: The Vigil and/or the new Spirit Slayers supplement (which isn’t required for the use of this SAS, but it helps). Under The Skin can be a "one-shot" adventure, a piece of an ongoing chronicle or the beginning of one. The experience point level should be appropriate for characters to whom the Vigil isn’t new, but who haven’t quite become grizzled veterans either.