After her father's death, Elizabeth (Louise Salter) visits a desolate island to discover why he left his money to a secretive order of nuns. Though the foreboding Mother Superior (Mariya Kapnist) tries to send her away, Elizabeth soon discovers that the convent is actually a prison, built to confine a terrible aquatic demon that can be controlled only if the shattered pieces of an ancient stone amulet are kept apart.