I barely knew Christine when she turned up at my door at around eight o'clock on the night of Halloween. We'd met for the first and only time three months earlier when my two roommates and I signed the lease on our apartment: Christine's aunt owned the place we were moving into, and she happened to be up from Delaware visiting at the time. But we'd only spent about five minutes together that day and we hadn't spoken much, and I hadn't thought of her since. Yet here she was standing outside my door with a friend. And both of...