PART 1: THE FALL

When Caroline Mercer woke beneath the harsh emergency-room lights in Philadelphia, the first thing she felt was pain beneath her ribs, and the second was terror for her seven-year-old daughter, Sophie.

Only two hours earlier, Caroline’s husband, Nathan Mercer, had shoved her near the top of the staircase inside their Main Line home. She remembered the sudden loss of balance, the edge of a step striking her back, and Nathan’s mother, Eleanor Mercer, standing above her instead of calling for help immediately.