In 1879, Irish-born, wandering rogues, Sean Mulcahy and Michael Lonergan, arrive in Monterey, California, where their past catches up with them and threatens their futures. They are two of the last surviving members of the St. Patrick’s Battalion, or San Patricios, an artillery unit in the Mexican Army during the Mexican-American War, made up of Irishmen driven to desertion from the American Army. After the war, the pair wander the American West, surviving saloon brawls and shootouts with bandits, until they come across an eight year old Chinese girl, left alone on a sand dune beside the bodies of a Chinese family, brutally murdered. They take the little girl to the city marshal of Monterey, who turns out to be an old comrade from the Mexican Army, Roberto Fuentes. He leaves the girl, Shushu, with the Lau family, who run a general store in the Chinese district. Roberto then introduces Sean and Michael to Monterey County Sheriff Fred Lang, State Senator Walter Maguire, and Walter’s sister, Olivia Connolly, who is president of a bank. Sean takes Olivia to dinner, where they clearly enjoy each other’s company. Later, Olivia learns her son, George, and his two friends had killed the Chinese family on the beach. Sean romances Olivia and strikes up a friendship with the Monterey parish priest, a Breton Franciscan friar, Father Lucien, while Michael begins to spend time with the Fuentes’ Chinese housekeeper, Ying. Shushu is treated cruelly by the Laus and runs away. Michael adopts Shushu, which becomes a topic of gossip in the town. George and his two friends grow concerned that Shushu might have seen them kill the Chinese family on the beach. While Olivia tries to talk Sean into helping her protect her son, the three young men confront Michael, forcing him to flee, along with Ying, and Shushu, and hold up in the ruins of the Carmel Mission, where they are joined by Sean for a climactic shootout with a posse, led by Sheriff Lang and Senator Maguire.