Nick and Allie are involved in a car accident and they die. Their souls however do not get to where they are going. They end up in a place called everlost, a place halfway between life and death. When they first arrive Nick and Allie meet up with Lief, a boy who has been in everlost for a while now. Together they travel to a place where MaryHightower was the "leader" of many Greensouls who had come to everlost. They lived together in harmony doing the same thing over and over again. Allie doesn't want to do the same thing over and over for the rest of her afterlife so she goes in search of someone called The Haunter. The Haunter can teach people how to haunt the living world. He gives Allie, Nick, and Lief a test and only Allie passes. With Nick and Lief captured Allie goes back to Mary for help, but doesn't get any. She then goes out on her own to find Nick and Lief which leads her to encounter the McGill, the monster of everlost. She starts to teach him how to skinjack (take over a living persons body), but that gets her put with the rest of the souls he has collected when he finds out that it can't be taught. Nick gets free and with Allies help gets off the ship and goes to get Mary to help free the captive souls on board. If you like Elsewhere by Gabrielle Zevin then you'll like this book too.