Eccentric scientist William Legrand lives on a remote island in South Carolina with his servant, Jupiter, a freed slave. Legrand moved to the island in shame after his family lost their fortune, which he is determined to restore. His hopes rise when he spots a beetle that appears to be made of solid gold. Both Jupiter and Legrand's friend, the unnamed narrator, believe a bite from the gold bug has driven Legrand insane. Legrand manipulates their worries, tricking them into helping him unearth buried pirate treasure.
The unnamed narrator is as much a mystery to the reader as any other element of the story, and in some respects, perhaps more so. The reader is given no description of the narrator or real insight into his character other than he believes Legrand to be insane. His primary purpose appears to be to recount the events of Legrand's discovery of the buried treasure, asking Legrand questions to elicit background information, especially how he solved the cipher, and what the coded message means.
Jupiter is a loyal, freed slave, who decides to travel with his old master, Legrand, to an isolated island to work as his valet and servant. Jupiter's misunderstandings of his master appear to be comic, but they point to the broader ways in which the information Legrand possesses is misunderstood by all of them when not seen in the proper light. Modern readers are apt to interpret his behavior and dialect, in many cases correctly, as racist representations.
William Kidd, better known as Captain Kidd, is the infamous pirate who buried the treasure Legrand digs up.
A fellow scientist on the island, Lieutenant G is equally taken with the gold bug Legrand discovers.