Plot

Buffoonish Ernest and his dimwitted pal Abner unearth a huge cannon reputed to contain the crown jewels of England.

Ernest P. Worrell becomes a new adventurer along with his History Professor partner, Abner Melon. Once they team up together, they discover a really huge existing historical cannon and the crown jewels hidden near the jungle. Ernest must save the jewels especially, before a lousy mob keeps it.—Anonymous

That lovable know-it-all knucklehead Ernest P. Worrell (Jim Varney), who is working as a janitor at a local college, meets a history professor named Dr. Abner Melon. After discovering an antique metal plate near a construction site, Ernest shows it to Dr. Melon who believes that it came from a giant American Revolutionary War cannon called "Goliath", (named after the legendary biblical giant). Dr. Melon had previously been ridiculed by his peers for theorizing that the real Crown Jewels of England were stolen during the Revolutionary War and were actually hidden inside the long-lost cannon.

Ernest and Dr. Melon begin to search for the artifact near the construction site and eventually locate it inside an abandoned mine. Unfortunately, they are ambushed by historical antiquity collector and Dr. Melon's colleague Dr. Glencliff whom they then lead on an action-packed chase through the countryside. Things become more complicated for them when British authorities hear about the incident and send a team of secret agents after them to retrieve the jewels. Dr. Melon's wife, Nan, on the other hand is only after him and Ernest for the jewels. While everyone is hot on their trail, Ernest develops a deep friendship with Dr. Melon.

After crashing the cannon into a forest, Ernest locates the jewels, not in its barrel as the legend describes but in the gunpowder kegs. After putting the crown on his head, he finds himself unable to get it off. Dr. Glencliff shows up, abducts him, and takes him to his clinic in an attempt to surgically remove it and kill him at the same time. Dr. Melon meets up with Nan and convinces her to help him save Ernest.

While at the clinic, Ernest manages to escape from the surgery room and lead Dr. Glencliff on a chase through the building. When he has nowhere else to hide, he and Dr. Glencliff have one last fight to get the crown before Dr. Melon arrives with the police. At the last minute, Dr. Glencliff takes an axe off of the wall and attempts to behead Ernest. Just as he is moments away from death, Dr. Melon bursts through the door and hits Dr. Glencliff over the head with the same antique metal plate Ernest found, knocking him out. Ernest realizes that Dr. Melon has saved his life and they both realize that they have gone from being adversaries to friends.

At the same time, British authorities arrive and explain to Ernest that the crown must be taken back to its rightful home. He explains that it won't come off his head and the authorities declare that whoever wears it is King of England. Fortunately for him (and England), Dr. Melon removes it for him by joking him about what is on his shirt, flicking him in the face. It causes it to fall off his head while everyone laughs. This is followed by him giving Dr. Melon a noogie while they argue about who would make a better king of England. The film ends here with a not-so-subtle moral, being that friendship is more valuable than status or material wealth.