Murder in the Air (aka The Enemy Within) is a 1940 American drama film with science fiction elements directed by Lewis Seiler and written by Raymond L.

[1] The film stars Ronald Reagan, John Litel, Lya Lys, James Stephenson, Eddie Foy, Jr., Robert Warwick and Victor Zimmerman.

Gabby Watters (Eddie Foy, Jr.), Brass's assistant, finds a letter in the dead spy's shoe, addressed to Joe Garvey (James Stephenson), the leader of a group being investigated by the House Un-American Activities Committee.

An unwitting Garvey assigns Bancroft to board the U.S. Navy dirigible USS Mason, which carries the newly invented "Inertia Projector", a sort of death-ray that can shut off an engine.

(The 'inertia projector' is an instrument which fouls electric current at the source; its amazing practicality is illustrated when it is focused on the plane in which the enemy agents attempt to flee the country.)