It features the masked hero "The Black Commando" facing Nazi saboteurs, inspired by Republic Pictures' successful Spy Smasher serial of the same year.

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.

Finally, Barton steals the formula and is captured by Thyssen and put under the protection of the sabotage ring.

Cline describes this as "propaganda in its basic form...delivered in the most effective way possible - by a respected authority figure in the person of one of Hollywood's most credible actors.

The Secret Code was released in Latin America in May 1944, under the title La Clave Secreta, in English with Spanish subtitles.