Crane's Colossal Carnival receives a radical downsizing after rubes rise up and the carnival's assets are repossessed, leaving ace pitchman "Honest" Bill Hogan (Robert Armstrong), owner Jeffrey Crane (Edmund Gwenn) and Crane's daughter Betty (Betty Furness) in desperate need of funds. Venturing to the local bank, Bill encounters aptly named Elmer Lamb (Stuart Erwin), a country bumpkin possessed of an extraordinary brain. Capable of amazing feats of calculation, Elmer works as a human adding machine, trying to earn enough money to buy the dairy farm of his dreams. Bill quickly dubs Elmer "Chain Lightning" and drafts him into showman service. Alas, Bill's best-laid plans prove a bust, but things turn around thanks to a bridge game aboard a train, where Elmer bests famed bridge champion J. Montgomery Brantley (E.E. Clive) alongside low-grade gangster Pudgy Murphy (Edward Brophy). But Pudgy has his own plans, and it might be time for Chain Lightning to fizzle out.