Paul Kersey is an architect, who is a peaceful man. But when his wife and daughter are attacked in their apartment, his world changes. His wife is killed and his daughter is in catatonic state. His boss decides that Paul needs to get out of New York so he sends him to Arizona to meet with a client. While their the client, a gun enthusiast gets Paul interested in guns and even gives him one. When Paul returns, he brings the gun with him when he goes out and when a mugger tries to hit him Paul shoots and kills him. After that he goes on killing spree. The police of course are investigating it, but the public feels thankful that someone out there is taking a stand.
