When FiM first aired, Celestia closed all portals connecting our worlds. We hadn't noticed them before the show aired, but the hopeful wishes of the bronies reached Celestia's ears and she reluctantly reopened them. But now she is having second thoughts. Ponies have gone missing as humans have started noticing the strange new world open to them, but humans as well are turning up dead and mutilated in Equestria and nopony knows where to point their hooves.