Zorrilla's Don Juan Tenorio is a Romantic transformation, or re-think, of the basic Don Juan story. It follows the basic outlines of the narrative used by others in the previous two hundred plus years, but it adds important elements and changes the overall thrust of the play and its moral.
Part I of the play contains the five-act structure of a complete drama. Don Juan is presented, from the start, as the legendary seducer who, as in Mozart's operatic version of 1787, keeps a list of the women he has seduced. But in Zorrilla's version, Don Juan is shown in opposition to another man, Don Luis, who seemingly wishes to compete with Don Juan in "immoral" attempts and makes a wager with him to this effect.
Zorrilla's point here seems to be that other men envy Don Juan and wish to be like him to beat him at his own game. In this regard, Zorrilla is a prototypical Romantic writer. The tendency of nineteenth-century literature is to exalt characters who are individualistic rebels and who defy the norms of society and the laws of man and God. In versions of the Don Juan story predating the nineteenth century, this view of the character may have been expressed but not explicitly—partly because of censorship and partly because artists themselves had not yet adopted the defiant and anti-religious mindset that would later characterize the Romantic period.
In this wager between Don Juan and Don Luis, it's not at all surprising that Juan comes out on top, though the outcome ends up being Juan's undoing. Don Juan is meant to be seen as both despicable and admirable. Through a ruse he gains entry to the apartments of Doña Ana, the betrothed of Don Luis. That his other seduction attempt of the night takes a different turn is evidence of Don Juan's dual-sided character, which, in keeping with the Romantic ethos, is a combination of good and bad, unlike the mostly good-for-nothing seducer of Tirso de Molina's and Moliere's treatments in the seventeenth-century.
Don Juan becomes overwhelmed with the purity and beauty of Doña Ines, a novice, or nun trainee, who is the daughter of the Commander of Seville, Don Gonzalo. It's as if his previously diabolical nature now has the potential to become angelic, and he waxes rhapsodic over Ines, declaring his great love for her. This, however, precipitates tragedy: it is as if Don Juan's ability genuinely to love a woman results, ironically, in his destruction. Or, Zorrilla's message is, conversely, that one's past will always catch up with one. Don Luis wants revenge for Juan's having seduced Luis's fiancee Ana, and Don Gonzalo disbelieves, unsurprisingly, that Juan has "honorable" intentions towards his daughter Ines. Don Juan, then, is forced to kill both Gonzalo and Luis.
This development differs from the similar point in the previous versions of the story. The chief plot device in de Molina, Moliere, and in Mozart's opera is the killing of the Commander alone, whose ghost, in the form of a statue, then exacts vengeance upon Don Juan. By having two killings, Zorrilla is, arguably, representing the good and evil sides of Don Juan and of human nature. Don Luis confronts Don Juan and duels with him in order to avenge one of Juan's typical, ruthless seductions. However, it's Don Juan's "conversion" to one who is actually capable of loving a woman that forces him into conflict with Don Gonzalo. Juan is forced to flee from Spain, and Ines dies of grief, setting the stage for Part II in which the supernatural phase of the story takes place.
We can see that Zorrilla's Don Juan is similar to other literary characters of the age, such as Heathcliff in Emily Bronte's Wuthering Heights and Julien Sorel in Stendhal's Le Rouge et le Noir. He has at least as much in common with these characters as with the Don Juan of Byron's epic poem, since, though one can see Byron's general influence on Zorrilla, Byron's Don Juan character and treatment of the story differ so much from the traditional version that there would have been little in them for Zorrilla to take as direct points of departure. Zorrilla prefers to stick to the basic outline of the legend and most of the literary versions. Probably, the most immediate antecedent for Zorrilla's version is Mozart's opera, which fired the imagination of the Romantic generation. Mozart's hero-villain has been interpreted by various commentators and writers (such as E.T.A. Hoffmann) as the prototype of the Romantic rebel, who, though on the surface he appears a cynical, manipulative seducer, is actually a kind of existential superhero who restlessly searches for the ultimate woman with whom he can bond. This is exactly the way I would interpret Zorrilla's Don Juan. In Ines he finds this idea, ultimate partner; she is the one who remains chaste, and rather than relinquish her, Don Juan confronts her father and kills him. Though at this point the reader, or audience, has no way of anticipating the outcome, in Part II it leads to Don Juan's salvation. As in the earlier versions, the animated statue of the Commander intends to drag Don Juan to hell, but the ghost of Ines intervenes and redeems him. The ending also has something in common with that of Goethe's Faust and its verse that "the eternal feminine leads us on high."
In summary, Zorrilla's treatment of the story is a Romantic re-think, an interpretation not grounded in the concepts of sin and guilt but in the dual, or multi-faceted nature, of human beings, who contain within themselves both good and evil. Don Juan makes the choice, unexpectedly, and perhaps unrealistically, to abandon his life style of seduction and crime when he encounters Ines. Both his flaws, and the better qualities within him, lead him to this, and the resulting tragedy of his killing both Gonzalo and Luis has the effect of both destroying and saving Don Juan. It is a typical Romantic juxtaposition of opposites, one in which the complexities and unexpected qualities of human nature are to be celebrated, and are the elements that enable man to strive towards the divine and the eternal, as the Romantic artist so often has him do.