Aboard a train, Marines Steve Riley, Mac McKay, Gunner Brady and Tennessee come to the defense of a young orphan boy, Ulysses Simpson Smith, on his way to visit his uncle in San Diego, California. Upon arrival in San Diego, the Marines and Ulysses find his uncle gone, and his apartment occupied by Molly Malone. After walking by the dreary orphanage, Steve takes Ulysses back to the base, where he is adopted as their mascot. Ulysses is outfitted with a dress uniform, and he and Steve go to visit Molly. Molly introduces her boyfriend Larry Allen, and they all eat cake. Afterwards, Steve invites Molly to visit the base and watch a parade. Molly attends the parade, and later as she and Larry walk around the base, he tells her that he plans to retire as soon as his enlistment is up and get married. Later at a landing practice, Ulysses ignores the gunfire in order to be with his friends but he is wounded when he saves Steve from an explosion. Molly and Larry plan to be married and they ask to adopt Ulysses, but he is determined to stay with Steve. Colonel Gage asks Steve to lead a secret mission to save an island that Steve is familiar with from enemy infiltration. Initially, Steve refuses, but when he visits Ulysses at the hospital, he hears the young boy's patriotic mumbling and decides to take on the mission. As Steve leaves Ulysses in Molly's care, he contemplates the motto on his Marine ring, "Always faithful."