After the death of her American father and Mexican mother, Justina Howland lives in Mexico with her mother's brother, who plans to marry the girl to his son in order to gain control of the rich mines that the orphan will inherit. To avoid the marriage, Justina runs away and starts for the home of her Aunt Elizabeth who lives in Lowell, Massachusetts. Detained at the border by American troops, Justina is befriended by Lieutenant Thomas Morgan, who helps her to reach her aunt's house. The uncle pursues her to Lowell, and in order to escape being taken back to Mexico, Justina disguises herself as a boy scout and joins the troop, now under the command of Lieutenant Morton, who has also come to Lowell. When the uncle overtakes the scouts and tries to force Justina to go with him, he discovers that he is too late because Justina has become Mrs. Thomas Morton.