Hired to investigate the murder of an 84-year-old widow, P.I. Joe Coopersmith hits one dead end after another in his search for leads. With few clues and no suspects, he nearly gives up, until he uncovers a connection to a bizarre plot to kill the descendants of Irish soldiers who fought for Mexico during the Mexican-American War. Known as San Patricios, they belonged to the St. Patrick's Battalion, an elite Mexican unit composed mostly of Irish Immigrants. When someone threatens Coopersmith's life, he knows he's on the right track. Undeterred, he digs deeper into the plot, soon learning the identity of the man behind it and his warped motive for the cold-blooded murder of the elderly widow.