She began to pull herself toward the top of the ramp while the Arrival fought with the empty wheelchair. Still struggling with only one wheel on the ramp, the car tried to push the chair, which had now toppled over onto its side, out of its way.

The chair collapsed, one of its wheels disappearing beneath the car. As the Arrival tried another burst of speed, it rode up on top of the chair, and the one wheel that had contact with the ramp was now also in the air, spinning.

The Arrival gunned its electric motor, the two front wheels rotating madly. But for all its efforts, it wasn’t getting anywhere. The more its motor whirred, the more the car actually appeared to be frustrated. Angry, almost.

Now Lillian had the time she needed to drag herself to the top of the ramp and onto the porch. The door was several feet away. She believed she was out of immediate danger, but knew she’d feel even safer once she was inside the house.

Instead of dragging herself, she now rolled, as she might have if she were on fire, although more slowly. She got to the screen door, wedged her fingers into the gap at the bottom, and swung it open.

That was when she heard: “Mom!”

She looked through the vertical posts of the porch railing and there, on the sidewalk, jumping off a bicycle, was her son.

“Mom!” Joe Bridgeman cried again.

The Arrival’s weight had now crushed the wheelchair, and its right wheel was back on the ramp. But instead of pushing forward, it reversed and got back onto the driveway, pointing itself toward the street.

There was now a new target.

“Look out!” Lillian screamed, her voice hoarse and weak.

Joe hardly needed the warning. He could see the Arrival had him in its sights. He thought, briefly, that he needed a red cape to wave before its electronic eyes. The car was a bull waiting to charge.

Joe took out his gun from the holster strapped around his waist and aimed it at the car. But what the hell were you supposed to say to a driverless car when it was getting ready to attack? Freeze? Drop it? Wheels up? Put it in park?

Joe settled on, “Don’t even think about it.”

The car did think about it, but not for long. Like a runner blasting out of the starting blocks, it came toward him.

Joe fired.

With people, you aimed for body mass. If someone was coming at you, gun or knife in hand, you didn’t aim for a leg or an arm. The chances of making that kind of shot were remote. You aimed for the body.

So Joe was not going to aim for a tire. Even if he were that good an aim, the car would probably keep on coming, riding on the flat.