When scientist John Townsend is kidnapped by the villainous Black Hand, he refuses to reveal the secrets that the criminal wishes to know. To convince him to talk, the villain abducts the scientist's spoiled, irresponsible son, Tom. The elder Townsend still refuses to divulge the information and, in a fit of rage, is strangled by the Black Hand. As he then turns to young Tom, a huge eagle comes crashing through the window of the dilapidated mansion. Floundering about, it happens to latch onto Tom's pants and flies off with the child in its talons.
Depositing him in its aerie, the eagle feeds Tom and nurses him back to health. Inspired by the fresh air, the youth puts himself through a rigorous physical regimen, developing "the muscles he had allowed to degenerate." One day, the eagle returns to the mountaintop with an American flag. Taking that as an omen, Tom whips it into a costume and battles crime as Captain Flag, helped along by his feathered benefactor who he names, Yank.