Mystery Plane is one of the better-known entries in Monogram's "Tailspin Tommy" series, if only because of its frequent TV showings. In this one, aviation ace Tommy (John Trent) and his pal Skeeter (Milburn Stone) develop a revolutionary new bombsight which they hope to turn over to the Army Air Corps. But a gang of international spies would also like to get their mitts on the invention, and to that end they exploit the friendship between Tommy and his on-the-skids mentor Brandy (Peter George Lynn), now employed as the villains' pilot. An outsized climactic fistfight and the ultimate regeneration of Brandy bring the story to a rousing conclusion. In a charming fadeout bit, the main characters-Tommy, Skeeter and girlfriend Betty Lou (Marjorie Reynolds) come "out" of the picture to invite their fans to watch the next "Tailspin Tommy" entry.