Farmer Giles ("Ægidius Ahenobarbus Julius Agricola de Hammo") is not a hero. He is fat and red-bearded and enjoys a slow, comfortable life. But a rather deaf and short-sighted giant blunders on to his land, and Giles manages to ward him away with a blunderbuss shot in his general direction. The people of the village cheer: Farmer Giles has become a hero. His reputation spreads across the kingdom, and he is rewarded by the King with a sword named Caudimordax ("Tailbiter") — which turns out to be a powerful weapon against dragons.
The giant, on returning home, relates to his friends that there are no more knights in the Middle Kingdom, just stinging flies — actually the scrap metal shot from the blunderbuss — and this entices a dragon, Chrysophylax Dives, to investigate the area. The terrified neighbors all expect the accidental hero Farmer Giles to deal with him.
The story parodies the great dragon-slaying traditions. The knights sent by the King to pursue the dragon are useless fops, more intent on "precedence and etiquette" than on the huge dragon footprints littering the landscape. The only part of a 'dragon' they know is the annual celebratory dragon-tail cake. Giles by contrast clearly recognises the danger, and resents being sent along to face it. But hapless farmers can be forced to become heroes, and Giles shrewdly makes the best of the situation.
Tolkien, himself a linguist, sprinkled several linguistic jokes into the tale, including a variety of ingeniously fake etymologies. Almost all the place-names are supposed to occur relatively close to Oxford, along the Thames, or along the route to London. Tolkien insists, tongue in cheek, that the village of Thame originally referred to the Tame Dragon housed in it, and that "tame with an h is a folly without warrant." Another joke puts a question concerning the definition of blunderbuss to "the four wise clerks of Oxenford" (a reference to Chaucer's Clerk; Tolkien had worked for Henry Bradley, one of the four main editors of the Oxford English Dictionary):
A short gun with a large bore firing many balls or slugs, and capable of doing execution within a limited range without exact aim. (Now superseded, in civilised countries, by other firearms.)and then satirises it with application to the situation at hand:
However, Farmer Giles's blunderbuss had a wide mouth that opened like a horn, and it did not fire balls or slugs, but anything that he could spare to stuff in. And it did not do execution, because he seldom loaded it, and never let it off. The sight of it was usually enough for his purpose. And this country was not yet civilised, for the blunderbuss was not superseded: it was indeed the only kind of gun that there was, and rare at that.
Chrysophylax comes across as a pompous aristocrat — rich, vain, and arrogant, but not actually malicious. Farmer Giles learns that he can be bullied, but is smart enough not to push him to desperation.