Alfred Kelly, a recent immigrant from Ireland, finds himself fighting Mexicans in the Mexican American war, a.k.a. Mr. Polks war. Distraught at his poor treatment by his Protestant fellow soldiers and officers and disgusted by their brutal handling of the Mexican people, he goes over to the other side, joining the Irish San Patricio artillery brigade. There, he fights against the Americans alongside the Irish leader, Jon Reilly. Captured, he barely escapes with his life and goes on a harrowing trip back to New York to rejoin his beloved, Erin. His return voyage is marked by many adventures, including narrowly escaping, once again, with his life. Taken in by Mexican guerrillas, he and the partisans are discovered by the Texas Rangers where many Mexicans are killed. He escapes by running into the jungle. Alfred eventually makes his way back to New York, leaving the Wicked War (as characterized by President U. S. Grant) behind him.