The author begins by saying that since his own age cannot supply a suitable hero for his poem, he will use an old friend, Don Juan. Don Juan was born in Seville, Spain. His parents are Don José and Donna Inez. Donna Inez is learned and has a good memory. Her favorite science is mathematics. She has a smattering of Greek, Latin, French, English, and Hebrew. Don José has no love for learning or the learned and has a roving eye. As his wife is rigidly virtuous and as he is incautious by nature, he is forever getting into scrapes. Consequently, there are quarrels between the two. Donna Inez, with the help of druggists and doctors, tries to prove that her husband is mad. She also keeps a diary in which she notes all his faults and even searches through his trunks of books and letters looking for evidence to use against him. Their friends and relatives try to no avail to bring about a reconciliation; their lawyers recommend a divorce. But before the situation can reach a critical point, Don Jose dies.
Donna Inez makes herself responsible for the supervision of Don Juan's education. He is taught riding, fencing, gunnery, how to scale a fortress, languages, sciences, and arts. His education is to a certain degree impractical, for he is taught nothing about life and studies the classics from expurgated editions. In short, his mother sees to it that he receives an education calculated to repress all his natural instincts and keeps the facts of life from him.
Among Donna Inez's friends is Donna Julia, a beautiful, intelligent young woman with Moorish blood in her veins. She is married to Don Alfonso, a jealous man more than twice her age. Theirs is a loveless marriage. It is rumored that Donna Inez and Don Alfonso had once been lovers and that she cultivated the friendship of Donna Julia to maintain the association with the husband. Donna Julia has always been fond of Juan, but when he becomes a young man of sixteen, her feelings toward him change and become a source of embarrassment to both of them. Juan does not understand the change that is taking place in him, but the more sophisticated Julia realizes that she is falling in love with Juan. She resolves to fight her growing love and never to see Juan again but the next day finds a reason for visiting his mother. She then convinces herself that her love is only Platonic and persuades herself that it will remain that way. Juan meantime cannot understand why he is pensive and inclined to seek solitude.
One June evening Julia and Juan happen to be in a bower together. One of Julia's hands happens to fall on one of Juan's. When the sun sets and the moon rises, Juan's arm finds its way around Julia's waist. Julia strives with herself a little, "And whispering 'I will ne'er consent'-consented" (St. 117).
As Julia lies in her bed one November night, there arises a tremendous clatter. Her maid Antonia warns her that Don Alfonso is coming up the stairs with half the city at his back. The two women have barely enough time to throw the bedclothes in a heap when Don Alfonso enters the room. Julia indignantly asks Alfonso if he suspects her of wrongdoing and invites him to search the room. Alfonso and his followers do so and find nothing. While the search is going on, Donna Julia protests her innocence with angry eloquence, giving numerous examples of her virtue and pouring abuse upon her luckless husband. When no lover is found, Don Alfonso tries to excuse his behavior but only succeeds in drawing sobs and hysterics from his wife. Alfonso, shamefaced, withdraws with his followers and Julia and Antonia bolt the bedroom door.
No sooner has Alfonso gone than Juan emerges from beneath the pile of bedclothes where he has been hidden. Knowing that Alfonso would soon be back, Julia and Antonia advise Juan to go into a closet. Hardly has Juan entered his new hiding place when Alfonso returns. Alfonso makes various excuses for his conduct and begs Julia's pardon, which she half gives and half withholds. The matter might have ended there had Alfonso not stumbled over a pair of men's shoes. He promptly goes to get his sword. Julia immediately urges Juan to leave the room and make his exit by the garden gate, the key to which she gives him. Unfortunately, on his way out he meets Alfonso and knocks him down. In the scuffle Juan loses his only garment and flees naked into the night.
Alfonso sues for divorce. Juan's mother decides that her son should leave Seville and travel to various European countries for four years. Julia is put in a convent from which she sends Juan a letter confessing her love for him and expressing no regrets.
The first episode of Don Juan ends at this point, but before concluding Canto I Byron adds twenty-two stanzas in which he entertains himself by giving a mocking statement of his intentions in regard to Don Juan, taunts his contemporaries Wordsworth, Coleridge, and Southey, defends the morality of his story, confesses that at thirty his hair is gray and his heart has lost its freshness, comments on the evanescence of fame, and says goodbye to his readers.
In the first few stanzas, Byron establishes the half-playful and mocking and half-serious tone that is going to pervade Don Juan. When that is done, he gives his readers as the chief characters in his first canto a pair of married couples. They are both unhappily married. Don José and Donna Inez are mismatched. Donna Inez is a cold and severe type of woman, although she has evidently not always been so. It was generally known that in her younger days she had had an affair with Don Alfonso. Don José is a good-natured, easy-going kind of man inclined to take his pleasures where he finds them. Byron's defense of him is that he had been badly brought up and that he was amorous by nature. In the character of Donna Inez, Byron was satirizing, against the advice of his friends, his estranged wife, Lady Byron. Donna Julia and Don Alfonso are mismatched by age as Donna Inez and Don José are mismatched by incompatibility of character and personality. Don Alfonso has nothing to offer Donna Julia except his name and station. Theirs was a marriage of convenience. Byron does not bother to devote much characterization to Don Alfonso. He merely says he was neither very lovable nor very hateable. He had a more or less negative personality, neither warm nor cold. Like any other husband, he did not care to be cuckolded.
Byron is far more interested in the wives than in the husbands and characterizes them rather extensively. Neither portrait is flattering. Donna Inez's is clearly malicious; in her Byron was attacking his estranged wife. She is not a faithless wife, but she is an intolerant and rather frigid one. Donna Julia's portrait of woman as wife is likewise unflattering; she deceives herself — and her husband. However, Byron makes the reader feel sympathetic toward her in spite of his using her to show up woman's wiles. Donna Julia and Don José, had they been closer in age, might have made a compatible pair; Donna Julia finds in Don José's son the warmth that was in the father. Donna Inez and Don Alfonso, who had been lovers at one time, might have gotten along well in marriage. Human nature and society, Byron seems to say, work against a happy marriage.
Some of Byron's contemporaries found Byron's bedroom farce immoral. It can be said in his defense that his mocking presentation neutralizes any remote occasion of sin that there might be present in his story of illicit love. Nor does he supply any provocative details. Lastly, both Donna Julia and Don Juan are made to look ridiculous, and both are punished for their guilt
The story in Canto I is told by an "I" persona who is said to be a friend of Don Juan's family. Byron may have foreseen the difficulties involved in making this persona a witness who would be present with Don Juan in his various adventures and so decided to discard him. At any rate the "I" narrator is discarded before the first canto ends, and becomes Byron himself giving his opinions on various matters and communicating more or less confidentially with the reader.
Canto I of Don Juan is without doubt the most interesting, entertaining, and amusing of all the cantos. For anything of this kind comparable in quality and liveliness in English verse, the reader has to go all the way back to Chaucer.