Miach Mihie's engineered suicide wave
After a nuclear-and-cancer catastrophe called the Maelstrom, a medical welfare state monitors every citizen through implanted WatchMe nanomachines. Miach Mihie, who as a teenager had led her friends Tuan and Cian in a starvation pact against that system, later hijacks the implants to drive thousands of people to kill themselves at once. The attack is designed to force the authorities to activate 'Harmony,' a program that erases individual will. Tuan hunts Miach down and kills her, but Harmony is switched on anyway, and humanity continues as smoothly functioning bodies with no one inside.
