Bob and Jackie Genofile live in the hills surrounding Los Angeles with their children. Los Angeles faces the Pacific Ocean on one side and the San Gabriel mountains on the other. McPhee notes that the San Gabriel mountains are disintegrating as the bedroom communities of Los Angeles push up against the mountains. One night, lightning struck the mountain. The Genofiles noticed a large, watery black mass—a debris flow in geological terms—rapidly approaching their home. The mass contained boulders and cars. Bob had built their house of steel and concrete to make it sturdy. However, the mass still smashed into their house and filled it up within six minutes, pressing the family into a bedroom. The family thought they would die, but the mud stopped just short of suffocating them. Usually, debris flows threaten property more than lives, and many people try to flee by car.

McPhee describes other well-known debris flows that have occurred in the greater Los Angeles area, including flows that have hit cemeteries, colleges, a plant nursery, and private residences. Residents build barriers known as deflection walls. They put up sandbags to protect against landslides. McPhee interviews Gary Lukehart—a resident in the area—and John McCafferty, who works as a bulldozer for hire.