I'm not sure if you are looking for a summary of the whole poem or just its conclusion, so I'm going to summarize the whole poem, and then you can pick from it whatever is useful to you: the whole or just a part.
A duke discusses a portrait of his former wife (who is now dead) with the servant of a man whose daughter he now wishes to marry and make his new duchess. The duke keeps the portrait behind a curtain so that no one can look at it without his permission. The painting was done by Fra Pandolf, and it is a good likeness, including the blush upon her cheeks. The duke describes his wife as one who was made happy by everything, no matter how large or small, and he disapproved of this because he wanted her to appreciate his gifts of status and fortune more than she appreciated other, more modest gifts. However, he refused to speak to her about it because he did not want to lower himself to explain his injured feelings to her; he is too proud for that. Instead, he seems to have had her killed: he says he "gave commands; / Then all smiles stopped together." In the end, he invites the man to whom he's speaking to return to their party, and he points out some other art pieces he owns, as though the painting of his last duchess is just another object he possesses.