Jimmy Martin, king of the motorcycle speedsters, visits the Kingdom of Mandavia for a race. There he is persuaded to impersonate the king by a traitor, Rodolph D'Henri, who intends to annex part of Mandavia for neighboring Selmarnia. The real king is in jail. D'Henri's plot is successful until Martin falls in love with Princess Margaret of Alvernia. Then he discloses his true identity, releases the real king, exposes the traitor, earns knighthood, and marries Margaret.