Mechanic and race car driver Eddie Shannon is the sap in question, played by Mickey Rooney. He's great with cars, and can think about little else, and is a proficient driver — although in top-drawer film noir fashion, he lives his lonely life on a single bed, next to a chest of drawers covered in trophies, the only one of which we can read being inscribed with the award of having come Second Place.
At work Eddie is bullied, both for his height — this is Mickey Rooney after all, and everyone in the film, even his super-attractive girlfriend played by Diane Foster, is taller than him.