As the poem begins, the Duke is discussing a portrait of the deceased Duchess with the Count's envoy who is invited to sit in order to listen to her tale ("That's my last Duchess painted on the wall, / Looking as if she were alive"). "That's" is a well chosen word because the Duke has objectified his wife, even when she was alive. He mentions the painting was done by an Italian monk, Fra Pandolph, whom he suspects of flirting with his wife by possibly saying to her that her cloak covered too much of her pretty wrist ("Her mantle laps / Over my Lady's wrist too much"). In this way, the artist was able to capture the Duchess' captivating smile ("spot of joy"). His former wife's smile was seen by the Duke as a spot or stain of noble character which was given too freely to others while it should have been reserved for him alone.
In his long speech to the Count's envoy, the Duke makes it clear that his main grievance with his dead wife was that she readily showed fondness toward other men ("She had / A heart . . . how shall I say? . . . too soon made glad, / Too easily impressed; she liked whate'er / She looked on, and her looks went everywhere"). She viewed no more fondly his attention ("My favour at her breast") than she did a sunset ("The dropping of the daylight in the West").
She looked as favorably upon her husband as at an inferior who brought her a cherry branch or even at a mule ride around the terrace ("The bough of cherries some officious fool / Broke in the orchard for her, the white mule / She rode with round the terrace--all and each / Would draw from her alike the approving speech"). She was as grateful to others as she was to him for the "gift" of his time honored name which marriage bestowed on the Duchess ("She thanked men,--good; but thanked / Somehow . . . I know not how . . . as if she ranked / My gift of a nine-hundred-years-old name / With anybody's gift").
His excessive pride prevented the Duke from mentioning the Duchess' perceived faults to her ("Who'd stoop to blame / This sort of trifling? Even had you skill / In speech"). The monologue ironically reveals that the falsely modest Duke is, in fact, a skilled speaker, although he feels above directly schooling a wife in the proper social graces. He could never bring himself to stoop to tell his wife that her actions metaphorically missed the archer's target of allowable conduct (""Just this / Or that in you disgusts me; here you miss, / Or there exceed the mark").
His jealousy grew as she smiled at him no more that at others ("Oh, sir, she smiled, no doubt, / Whene'er I passed her; but who passed without / Much the same smile?') In fact, his envy raged to such an extent that he ordered her murder ("This grew; I gave commands; / Then all smiles stopped together"). Chilling. It is a sad commentary on society that one such as the Duke could be invested with such power that he could order the execution of his wife for not giving him all the attention he desired.
Having finished telling the Count's envoy the tale of his former wife, the two descend the staircase to meet the assembled guests below ("Will 't please you rise? We'll meet / The company below"). In the assembly hall of the palace, the discussion over the dowry he will receive for marrying the Count's daughter will presumably begin in earnest ("I repeat, / The Count your Master's known munificence / Is ample warrant that no just pretence / Of mine for dowry will be disallowed"). The greedy Duke disingenuously insists he is really only interested in his wife to be ("Though his fair daughter's self, as I avowed / At starting, is my object") "Object" is an apt word as that's how the Duke views any wife.
The poem ends with a reference to another item in the Duke's art collection: a statue of the Roman god of the sea breaking in a new sea horse to drive his chariot ("Notice Neptune, though, / Taming a sea-horse, thought a rarity, /Which Claus of Innsbruck cast in bronze for me"). The Duke hopes that his new wife will be properly broken in before he marries her. It explains his frankness with the Count's envoy who he hopes will discreetly convey his expectations of what is considered proper conduct for a Duchess. It is appropriate that the poem ends with an exclamatory "me" just as the title begins with "My" as the Duke only cares about himself. It is equally consistent with a disturbing poem on jealous possessiveness that it is framed around discussions of a painting at the start and a statue at the end. One is left wondering if the Duke's new wife will also be added to his art collection.