Because Tommy, the small son of Captain Phillips, who is experimenting with new explosives, can't seem to get much attention, the boy goes out to romp with his grandfather, a veteran of the Civil War. Tommy's sister Katie, and her fiancé, Dick, are sitting on the veranda of the Phillips home. Miklos Vitez, a foreign gentleman who is also a suitor of Kate's, joins them. Kate, to make Dick jealous, flirts with the foreigner. Dick, angry, leaves. Captain Phillips considers Vitez a friend, although the colonel, his best friend and confidant, warns him. Vitez learns where the captain keeps his notes and formulas. One day he steals the key to the laboratory and hides in a closet behind the stairs, while Janos, a foreign accomplice, phones the captain that he is wanted at the barracks. When Kate and Dick leave the house, Vitez furtively makes his way into the laboratory. Tommy has gone to bed, where the old man is relating war stories to the boy. The grandfather hears a noise, and ordering Tommy to go to sleep, goes downstairs with his old musket on his shoulder. Katie has seen the light in the laboratory and takes it for granted that her father has returned. The two start toward the house. Janos, hidden in the shrubbery, catches Kate, and he and Dick fight, Dick throwing the foreigner down the well. Then Kate rushes for a rope and they try to pull Janos from the well, but he is afraid to come up. The captain, meantime, having arrived at the barracks and being informed that there has been some mistake, becomes suspicious, and rushes home. The grandfather enters the laboratory to find Vitez working with the strongbox. He holds him up, but when the foreigner makes a move toward him, and he fires, no sound is heard, the musket being too old. Vitez springs upon the old man and begins to choke him. Tommy, awakened, goes to the laboratory, and picking up a bottle of explosive, hurls it at Vitez. The room is wrecked. When Kate, Dick, the captain and the policemen enter the room, they find Vitez dying. Tommy, though injured, is trying to help his grandfather from the laboratory. A week later, Tommy tells his grandfather that he knows he hasn't much chance of ever being a hero now that he is injured, but he changes his mind when the colonel, the captain and the sergeant visit him to present him with a medal, upon which is inscribed "To a Little Hero, from His Regiment."