Murder in the Air.

Enemy agents are everywhere and they are sabotaging all important war deliveries. The body of a hobo found in a train wreck had a money belt with $50,000 and a tattoo of a circle and arrow. This is a tattoo for saboteurs for hire and Brass must impersonate the dead man to find out what his orders are. As Steve Coe, he meets with the band of enemy agents in California and everything goes well until the wife of the dead 'Hobo' shows up. Luckily, Gabby is able to save Brass and Brass learns what is his assignment. He is to board the USN airship 'Mason', which is testing the super secret Inertia Projector, and destroy the airship.