He runs at the camera, lunges for her, catching her left ankle as it shoots out in her ungainly crab crawl. He drags her, kicking and thrashing, away from the clear focus that felt like safety, back into the open foyer. He pulls her leg up, tilting her onto her back. He leans in, stooping low to make a short fall of the distance left to be on her, to finally catch her under his control. And Carly Liddell, never a dancer, never a gymnast, never any color belt in any martial art—but ever the natural math and science whiz—becomes trigonometry and physics. And she has cool boots.

Her mind and muscles do the calculations of the arcs and angles as she rises up, torso cocked to the left, then swinging to the right to load momentum into her free leg, which she brings back across her body. The knobby tread of her goth-girl combat boot explodes his grip on her ankle.

In a perfect ballet of Newtonian inevitability, unlearned and unpracticed but as natural as a whirlwind, Carly makes a figure-eight flourish of the follow-through, winding up again, this time to bring her boot crashing into the sweet spot where his jaw meets his ear, dropping him like a bag of gravel.

She rolls onto her hands and knees. She pushes up from the floor and looks down at her fallen foe. Run! thinks every single person who will ever watch the footage.

And she does.

It’s magnificent.

But short-lived. The video freezes and the cybercrime tech destroys the triumph and tension with a quick electronic red circle, drawn to bring the audience out of the drama and into the lineup. The young man’s face, sideways in forced repose against the foyer tile, is largely in shadow and not terribly in focus, but it’s lit up enough that someone who knows him well might peg him. To the stranger, he still looks rough-hewn and indistinct. But viewer forty-four had already picked up her phone by the shot of him coming off the retaining wall. He was arrested just a few hours after Carly walked past him on the sidewalk.

It was a good day for Good Samaritans. It was a good day for law enforcement. It was a good day for the local news outlets that vied to make the most appealing special report of the pulse-racing video and happy ending.

And it would have been a good day for John Cooper. His elaborate security system, which his new wife teased him for, had caught the reckless and newly bold young man who had attacked his stepdaughter, and it got the boy before he’d done all the terrible things he’d been whittling vivid in his mind for years.

The system had worked just as designed, its clarity and clever placement revealing what had happened and when and how, and most importantly, by whom.