Secret Service of the Air (1939)

Illegal aliens are being flown in from Mexico by small plane. After a Secret Service agent finds out, he is killed and the smuggling is shut down for a while. Brass, an ex-Navy flyer who is now flying the Pacific Clipper route, is chosen by the Service to find out who is behind this smuggling ring. Since they know that an ex-smuggler named Ace was part of the gang, Brass is put into the general prison population to try to get information from him. It is from Ace that he learns where the smugglers are located, but it is up to Brass to infiltrate the ring and bring the head man across the border to be arrested.