A billionaire software-genius named Harold Finch creates a Machine for the government that is designed to detect acts of terror before they can happen, by monitoring the entire world by monitoring every cell-phone, email and surveillance camera. Finch discovered that the machine sees everything, potential terrorist acts and violent crimes that involve ordinary people. When the government considered violent crimes between normal people "irrelevant", Finch built a back door into the system that gives him the social security number of a person involved in a future violent crime so he could act. Partnered with John Reese, an ex-CIA agent, the two work in secret to prevent violent crimes before they can happen.
I feel sorry for anyone who isn't watching this masterpiece at the moment. Best show on network TV hands down.