That endearing and somewhat foolish janitor, Ernest P. Worrell, works at a local college and unexpectedly crosses paths with a history professor named Dr. Abner Melon. Their adventure begins when Ernest discovers an antique metal plate at a construction site. Excited, he shares his find with Dr. Melon, who theorizes it may have originated from a legendary American Revolutionary War cannon called "Goliath," a name inspired by the infamous biblical giant. Dr. Melon, who has faced skepticism for his belief that the true Crown Jewels of England were hidden within this long-lost cannon during the Revolutionary War, seeks to validate his ideas.
Eager to uncover the truth, Ernest and Dr. Melon embark on a quest that leads them to an abandoned mine near the construction site. However, their journey takes a treacherous turn when they are pursued by the nefarious antiquity collector and Dr. Melon's rival, Dr. Glencliff. Their escapade results in a breathtaking chase through the countryside, especially when British agents catch wind of their antics and set out to reclaim the jewels. Complicating matters further, Dr. Melon’s wife, Nan, has her own agenda, seeking the jewels for herself amidst the chaos.
As the frantic chase continues, an unlikely bond forms between Ernest and Dr. Melon, despite the peril surrounding them. After a disastrous crash involving the cannon in a forest, Ernest surprisingly discovers the jewels inside the gunpowder kegs rather than within the barrel as legend suggested. In a humorous twist, after placing the crown on his head, he finds it humorously stuck. Shortly thereafter, Dr. Glencliff captures him, taking him to his clinic with nefarious intentions to surgically remove the crown.
In a timely twist, Dr. Melon unites with Nan, persuading her to assist in rescuing Ernest. Meanwhile, at the clinic, an amusing but chaotic escape unfolds as Ernest leads Dr. Glencliff on a wild chase through the facility. Cornered and desperate, he confronts Dr. Glencliff in a comical showdown over the crown just as Dr. Melon, alongside the police, arrives to the rescue. In a climactic moment, just as Dr. Glencliff attempts to decapitate Ernest with an axe, Dr. Melon saves the day by using the very antique plate Ernest uncovered, knocking Dr. Glencliff out cold.
As British authorities arrive, they inform Ernest that the crown must return to its rightful place. However, he comically explains that it’s stuck on his head, prompting the astonished authorities to declare him the King of England by virtue of wearing it. In a light-hearted moment, Dr. Melon cleverly removes it by playfully distracting Ernest, causing the crown to plop off amid everyone’s laughter. This playful finale culminates in a friendly squabble over who would make a better king, leading to a heartwarming moral: the value of friendship far exceeds any status or treasure.
