Bestselling author Markus Heitz invites readers on an action-packed mystery adventure across three captivating alternate worlds - choose for yourself which world you want to experience first: "DOORS ?", "DOORS !" or "DOORS X"? 3 books, 3 worlds, 3 doors - which ones will you open? Behind these doors lies much, including adventure. Can someone disappear in a house? The clues left by Anna-Lena van Dam clearly lead to the long-abandoned villa of her great-grandfather - but not back out. Her father suspects she is in a secret cave system beneath the house and sends ex-soldier Viktor with a geo-expert team to search for her. The presence of a parapsychologist and a medium among the six people has a good reason, as Viktor will soon discover. Because the caves are not what they seem.