An ex-assassin and a wealthy programmer save lives via a surveillance AI that sends them the identities of civilians involved in impending crimes. However, the details of the crimes--including the civilians' roles--are left a mystery.

Harold Finch: [Opening narration from Season One] You are being watched. The government has a secret system, a machine that spies on you every hour of every day. I know because I built it. I designed the machine to detect acts of terror but it sees everything. Violent crimes involving ordinary people, people like you. Crimes the government considered "irrelevant." They wouldn't act, so I decided I would. But I needed a partner, someone with the skills to intervene. Hunted by the authorities, we work in secret. You'll never find us, but victim or perpetrator, if your number's up... we'll find *you*.
Harold Finch: I'm a very private person.
Harold Finch: [Opening narration, Season 2] You are being watched. The government has a secret system, a machine that spies on you every hour of every day. I designed the machine to detect acts of terror but it sees everything. Violent crimes involving ordinary people. The Government considers these people "irrelevant". We don't. Hunted by the authorities, we work in secret. You'll never find us, but victim or perpetrator, if your number's up... we'll find *you*
[Repeated line]
Nathan Ingram: Everyone is relevant to someone.
Harold Finch: [Opening narration, Season 4] We are being watched. The government has a secret system, a machine that spies on you every hour of every day. I designed the machine to detect acts of terror but it sees everything. Violent crimes involving ordinary people. The Government considers these people "irrelevant". We don't. Hunted by the authorities, we work in secret. You'll never find us, but victim or perpetrator, if your number's up... we'll find *you*.
Mark Snow: [Repeated line] Don't forget, you're behind enemy lines.
John Reese: [When asked who he is] A concerned third party.