Little Kate and Janie O'Dowd are sent to their wealthy American uncle, Michael O'Dowd, after their Irish father loses his life on a World War I battlefield. Having been locked accidentally into O'Dowd's munitions plant one evening, the children catch sight of their intoxicated cousin Miles O'Dowd admitting two men into the factory. The girls recognize the two as spies they had seen on the boat to America sending signals to a German submarine. After the spies knock Miles cold, the children trap them in a die-stamping machine until help arrives. Miles and factory worker Jerry Flynn, who loves young Patricia O'Dowd, enlist and are soon joined by Alfred Vanderspent, whose wealthy mother's plot to falsify his birth records is foiled by the children.