Pretending to be a brave explorer, the feebleminded but well intentioned Ernest P. Worrell hunts for diamonds at a construction site. After maximizing the voltage on his metal detector, an electrical charge animates a nearby buzz saw that chases Ernest around the property and leaves the unfinished house in ruins. Meanwhile, J. Quentin University history professor Dr. Abner Melon is ridiculed by the Board of Regents for a paper proposing that England’s King George III sent the British Crown Jewels to Virginia during the Revolutionary War. He concludes that the items currently on display in the Tower of London are illegitimate, while the real gems remain hidden in Virginia. After the meeting, esteemed collector Dr. Radnor T. Glencliff encourages Abner to come to him with any further developments in his research. Outside, Abner runs into Ernest, who works as the university janitor. Ernest accidentally destroys an invaluable reference book, but Abner’s distress is short-lived once he notices Ernest holding a battlefield marker belonging to the cavalry unit responsible for stealing and hiding the British Crown Jewels. Believing the item contributes to the validity of his theory, Abner takes Ernest to see Dr. Glencliff, who insists he come up with more proof. Although his wife Nan disapproves of his association with Ernest, Abner realizes that Ernest is the only person who has faith in his theory. Together, he and Ernest return to the construction site and uncover a cannonball fired from “Goliath,” the largest cannon ever cast. The men follow the cannonball’s trajectory to a nearby grassy knoll. As Abner begins to lose faith, Ernest falls into the underground bunker containing Goliath. The cannon shifts and rolls downhill, destroying Nan’s new car. Ernest climbs into the cannon bore and becomes stuck just as Glencliff arrives to confiscate the artifact for himself. On the way back to Dr. Glencliff’s office, Ernest dislodges himself and releases the piece of artillery from its hitch. He rides the runaway cannon as it rolls through a family garden party, and Abner gives chase in a dune buggy. Narrowly evading the gunfire of Glencliff’s pursuing men, Abner jumps onto the cannon, which comes to rest in a barn. As the pair stops to eat, Abner thanks Ernest for his help and admits they have become good friends. They hitch the cannon to a tractor and continue their journey. When they reach a dead end, Ernest uses a blowtorch to fire a cannonball at Glencliff’s approaching car. Realizing the cannon does not hold the hidden jewels, Abner blames Ernest for their failure and storms off. Moments later, however, Ernest hits his head on the cannon chamber, revealing a secret compartment containing the British Sovereign’s Orb and crown. He shouts the news to Abner, but his yells attract Glencliff’s attention. Unable to wrest the crown from his head, Glencliff plans to cut open Ernest’s skull. Ernest escapes and runs to Glencliff’s artifact room, where a fight ensues. Meanwhile, Abner reunites with Nan and disappoints his wife by confessing he did not find the jewels. When Nan criticizes Ernest, Abner defends his friend and rushes off to save him. He barges into the office and knocks Glencliff unconscious. A British Secret Service member arrives to reclaim the jewels, but because the crown will still not budge from Ernest’s head, he declares that whoever wears the crown must be named rightful King of England. Claiming he will miss Ernest if he leaves, Abner pops the crown from his head, and the two men erupt in a fit of laughter.