As the morning sunshine descends upon Blandings Castle, the estate’s owner, Lord Emsworth, is playing with his new telescope and uses it to spy on his son, Freddie, who is in the embrace of a strange young woman. This revelation devastates Emsworth, who’d hoped his son would someday find an eligible girl belonging to a good family. After ambushing his son on the terrace, Freddie identifies her as Niagara “Aggie” Donaldson. She’s American, and a “sort of cousin” of the estate’s head gardener, Angus McAllister. Also, she’s his fiancée. These details outrage Lord Emsworth even further, and he rushes to confront McAllister himself. Emsworth tells the gardener that if he does not send the girl away, he will lose his position at the estate. McAllister responds to this threat by calmly handing in his notice. Lord Emsworth, who rarely considers the consequences of his actions, is initially pleased with this result, but later has a realization: without McAllister, who will take care of Lord Emsworth’s pumpkin?
The titular pumpkin is of great importance to Lord Emsworth, as it is the one vegetable his family has yet to win first prize for at the Shrewsbury Agricultural Show. Emsworth feels this “blot” on his family’s record deeply, and when his winning vegetable begins to droop, he realizes the gravity of his mistake. Emsworth sends a telegram instructing McAllister to return at once. McAllister responds that he will not. Emsworth, who had never considered the possibility that McAllister might refuse, reluctantly decides that he must go to London—a city he despises—to find a suitable replacement.
Lord Emsworth’s trip to the city, however, proves to be both unpleasant and fruitless, as he is unable to find even one candidate who matches his requirements. On the third day of his visit, he coincidentally bumps into Freddie, whom Emsworth had previously banned from London due to his habit of racking up tremendous amounts of debt. In an effort to avoid conflict, Freddie hands his father a note and quickly leaves. The note reveals that he and Aggie are now married. Lord Emsworth, delirious with shock, is overtaken by an urge to be among nature, and hails a cab to Kensington Gardens.
Once at the park, the well-set-out flowerbeds affect Emsworth something “like a drug.” The Lord enters a trance-like state and, believing himself to be back at Blandings, proceeds to commit a crime of unspeakable proportions: he steps over the railings and begins to pick flowers. This flagrant criminal act attracts the attentions of the park-keeper, followed shortly by a crowd of spectators and a police constable. When Emsworth claims to be an Earl, the crowd—who see him as nothing more than a strange man in an ill-fitting suit—deride him. Emsworth is eventually spared from this ordeal by the fortuitous arrival of McAllister and Mr Donaldson, the former of whom affirms the Lord’s identity. The onlookers disperse, and Mr Donaldson introduces himself as Aggie’s father. Despite Emsworth’s previous concerns that Aggie was not from a “good family,” it turns out that Mr Donaldson is an extremely wealthy industrialist. Not only this, he sees great potential in Freddie and intends to put the boy to work in Long Island City. Thrilled by this development, Emsworth tells Donaldson to inform Frederick that he has his father’s best wishes, and that there’s no need to hurry home. Emsworth then turns his attentions to McAllister, who he begs to return to the castle. McAllister agrees—though only once Emsworth doubles his salary.
At the Shrewsbury Show, Sir Gregory Parsloe-Parsloe, Emsworth’s rival, offers his congratulations to the Lord, while McAllister silently observes. The pumpkin sits in one of the largest packing-crates Shrewsbury has ever seen. The note attached reads, “PUMPKINS. FIRST PRIZE.”