Oh my word, Dan Poblocki does it again.  This time it is a series with an app connected to it - so cool.  In the book 5 kids are drawn to a mysterious house - 1 thinks it's to meet a long lost relative, 2 think it is a movie location, another a special music school, and the final thinks it is a boarding school.  All of the kids are escaping something - a dead sister, a life in an orphanage, etc.  But when the get to the house nothing is as it should be and they find themselves trapped.  As they are looking around they meet 5 other mysterious kids in strange masks.  Who are these kids and what happened here.  The reader learns more about the back story by interacting with the free app.  Genius and very creepy.