Abby Watson had been dead to the world when a familiar, and yet highly unusual, noise woke her. It sounded like firecrackers; it sounded like an assault rifle.

It took her a moment to realize she wasn’t dreaming, and even though it didn’t sound like it was coming from the hallway directly outside of her door, it seemed to be on this floor. She heard several more shots and screams of terror as she dug her gun out of her bag and hurried to the door.

She opened it cautiously, stuck her head out into the hallway, and saw that it was clear. Staying against the wall, she slid down...