Le jeune séminariste américain, Michael Kovak, se rend au Vatican pour y étudier les rites de l’exorcisme. Féru de psychologie, il nourrit de sérieux doutes à l’égard de ces pratiques anciennes et juge que la « possession » relève de la psychiatrie, plutôt que de la démonologie. Il se heurte périodiquement à ses formateurs, jusqu’au jour où ceux-ci l’adressent au Père Lucas, ecclésiastique légendaire, qui a pratiqué avec succès des centaines d’exorcismes. Au contact de ce mentor, au comportement abrupt et déroutant, Michael commence à se déprendre de ses préjugés. Un cas se présente bientôt à lui, dont la violence terrifiante va le forcer à se remettre en question…

Ferocious humanoid creatures from the future come back to the present to devour humans.

Six years after a violent car crash claimed her parents' lives, Jesse is still trying to put her life back together. The situation is complicated by the fact that her parents virtually vanished, as did the accident wreckage, so there's no evidence for the police and certainly no closure for her. Jesse meets her fears head-on when she and a group of filmmakers break into an abandoned mental institution, she immediately experiences a visceral deja-vu. The asylum seems somehow connected to her missing parents and furthermore, the building is far from deserted. The group discovers that a sinister doctor has taken up residence in the building's vast underground and he is using extreme science trying to bring his wife back from the dead! The basement also contains the countless remains of the doctor's failed experiments, all of which are now slowly coming back to life when they make contact with a murky substance that has begun to leak out of several holding tanks!