When his beloved only daughter goes missing, millionaire entrepreneur Walter van Dam calls in a team of experts - including free-climbers, a geologist, a parapsychologist, even a medium - to find her . . . for Anna-Lena has disappeared somewhere within a mysterious cave system under the old house the family abandoned years ago. But the rescuers are not the only people on her trail - and there are dangers in the underground labyrinth that no one could ever have foreseen.

 In a gigantic cavern the team come across a number of strange doors, three of them marked with enigmatic symbols. Anna-Lena must be behind one of them - but time is running out and they need to choose, quickly. Anna-Lena is no longer the only person at risk.

 The team knew their mission would be perilous - but how do you defeat your own demons? Trapped in their own nightmares, their only hope of escape is DOOR X, which leads to a threatening vision of the future . . .