There is an irony in it that I have never stopped turning over.

I installed that security system to protect my family from outsiders — from the burglar, the stranger, the threat that comes from beyond the walls.

Every dollar of it, every camera and every sensor, was pointed, in my mind, outward, at the dangerous world I imagined pressing in against my home.

It never once occurred to me, in two years of paying the monitoring bill, that the system’s most important function would turn out to be recording the people I had installed it to feel safe among.

The danger to my family, when it finally came, did not come through a jimmied window or a forced door.

It walked in through the front, with a key I had given it, wearing my mother’s face and speaking in my mother’s voice, and it did its worst work in the kitchen and the living room and at the thermostat on the wall — in the very rooms I had wired, not against her, but against a threat I thought I understood.