When Abner Peabody, the proprietor of the Jot 'Em Down general store in Pine Ridge, Arkansas, hears a radio appeal by Chester Marshall, the head of the Civilian Aid in War Effort Board, asking for the "common man" to develop new ideas and inventions, Abner sets up a chemistry lab in the store's basement and begins to experiment. When Abner discovers a formula for synthetic rubber, his partner, Lum Edwards, suggests that they travel to Washington, D.C., to personally present the compound to Marshall. Upon arriving in the nation's capital, Lum and Abner find that the city is plagued by an acute housing shortage, and consequently, when a stranger in the park offers to rent them a room for the night, they readily accept. The man guides them to their lodgings in the dark, and upon awakening the next morning, they discover that they are sleeping in the bedroom display of a department store window. After being ushered out of the window and into the store by the irate manager, Lum and Abner meet their old friend, newspaper columnist Robert Blevine, who is shopping at the store. Robert invites the two to stay at his house, and escorts them to Marshall's office, where thousands of citizens have come in answer to the appeal. At the office, Marshall's secretary, Jane Nestor, chides Robert for criticizing her boss in his column and refuses his dinner invitation. While waiting for an appointment to see Marshall, Lum and Abner tour the national monuments. Tired after a day of sight seeing, the pair rest on a bench in the park. When Lum overhears a senator discussing the problem of drought in his state, he suggests using worms to enrich the soil. Next, Lum advises a congressman about how to keep the small-town population of his district from migrating to the city. Word of Lum and Abner's homespun wisdom sweeps through Washington, and soon the city is flocking to the park bench to consult with the country sages. When Marshall appears at their park bench, Lum and Abner show him their synthetic rubber invention, and he calls a press conference to announce the discovery. At the conference, Abner is in the process of reproducing his compound when a statue topples and hits him over the head, inducing amnesia. When Abner forgets his formula and thinks that he is a "hep cat," the press ridicules Marshall. After the government tells Marshall that he must produce the formula in seven days or resign his post, he joins Lum and Abner as they return to Pine Ridge. On the final day of the government's ultimatum, Abner's memory is restored when he bumps his head on a shelf. Soon after, Jane arrives from Washington with news that government scientists have determined that Abner's formula is a compound for synthetic asphalt, not rubber. For their valuable contribution to the war effort, Lum and Abner are then appointed to head an advisory committee on farm problems, vindicating Marshall's faith in the common man.