The Secret Code (1942) was the 19th serial released by Columbia Pictures. It features the masked hero "The Black Commando" facing Nazi saboteurs, inspired by Republic Pictures' successful Spy Smasher serial of the same year. The chapters of this serial each ended with a brief tutorial in cryptography.

Plot

This serial introduces the World War II scenario when a masked hero tries to prevent Nazi agents from crippling the U.S.'s war effort. The spy ring is led by fifth columnist Jensen, who, with his lieutenant Rudy Thyssen and a network of Nazi saboteurs, is trying to get possession of a top-secret formula the United States had developed for manufacturing synthetic rubber while creating explosive gases and radio-controlled bombs to sabotage the exhausting war effort. Police Lieutenant Dan Barton stages a public dismissal from the police department in order to join the saboteurs ring and learn the secret code they have been using. To further assist his efforts (especially after his superior, the only person to know that Barton is working undercover, is murdered), Barton assumes the secret identity of the Black Commando, a masked man who is wanted both by the villains (who want the secret formula they think he has) and police (who are also searching for Barton for murder). Finally, Barton steals the formula and is captured by Thyssen and put under the protection of the sabotage ring. Joining the gang, he learns of their plans, which he immediately leaks to his best friend and former partner Pat Flanagan (and through Flanagan to reporter girlfriend Jean Ashley) and, as "The Black Commando", continually frustrates the Nazi plots. After innumerable dangers and lost efforts in trying to decipher the enemy's secret codes, Barton and Flanagan discover the key to the Nazi code, capture the Nazi ring and make sure that the Nazi U-boat which has been waiting to help the Nazis escape is depth-bombed and destroyed.