The American army is about to unveil their first jet fighter and the Nazis are determined to steal the plane and study it to build a jet fighter of their own. Meanwhile, Diana's younger sister Drusilla arrives from Paradise Island for a visit and gets caught up in the Nazis' plot. Nazi Kriegsmarine Captain Radl also wants to find out the source of feminum, the metal used to make Wonder Woman's bracelets. He kidnaps General Blankenship, who is taking Drusilla for a drive in Washington. Drusilla discovers that she can spin into costume like her sister and turns into Wonder Girl. Wonder Girl tries to rescue the General but Captain Radl chloroforms her.
Peter Knight, an engineer secretly working for the Nazis, gains Drusilla's trust and tricks her into revealing the location of Paradise Island. The Nazis stage an invasion and force the Amazons to mine feminum ore. Wonder Girl escapes and as Drusilla exposes Peter as a Nazi spy. Learning Diana has returned home, Wonder Girl returns to the island and she and Wonder Woman engineer the capture of all of the invading Nazis. Wonder Woman and Wonder Girl then return to America to prevent the theft of the jet.