1. S O S Coast Guard (1937) Ralph Byrd - 12 Chapter Republic Serial.
The plot concerns the mad scientist Boroff (Bela Lugosi) attempting to sell a super-weapon to the highest bidder, opposed by Coast Guard Lieutenant Terry Kent (Ralph Byrd), for both personal and professional reasons.
Boroff is a mad scientist who has invented a "disintegrator gas" and plans to smuggle it to his buyers in Morovania. When his ship, the Carfax, gets stranded on outlying rocks in the first chapter, the Coast Guard comes to rescue him. Recognised by the reporters, Jean and Snapper, Boroff runs and kills the pursuing coast guard Ensign Jim Kent, who turns to be Lt Terry Kent's brother.
As the gas is made from the rare substances Arnatite (which is radioactive) and Zanzoid, Boroff attempts to acquire more of these materials to create more (including salvaging supplies of arnatite from the sunken Carfax). Hot on his heels are the Coast guard, led by Lt Kent, and the two reporters, with the expert aid of Jean's chemist brother, Dick. Eventually Terry finds, and leads a squad against, Boroff's cave-based hideout, with disintegrator gas bombs exploding around them.