When the portrait is finished and Dorian looks at it, he realizes that it won't be long before he'll lose his radiant looks; like everyone else, he'll age and die. He wishes he could remain young, and that only his portrait would age.
His wish comes true, though he doesn't realize it at first.
As time passes, he falls under Wotton's influence and begins to live his life according to what he thinks are the older man's principles. He lives decadently, commits a number of unnamed sins, cruelly hurts a young actress, Sibyl Vane, who's in love with him, and commits crimes that break the law and violate people. All the while, he remains physically young and beautiful. His portrait, on the other hand, reflects the state of his age and character; it slowly turns into the likeness of an old and vile man.
One theme that runs through the book is people's views on art, and how their views affect the way they live. For Basil, life is a source of inspiration for art; he's entranced with Dorian and paints his portrait because he sees the young man as innocent and radiant, his personal muse. As the novel goes on, and Dorian becomes more evil and corrupt, Basil loses his ability to produce compelling work.
Unlike Basil, Dorian treats life itself as art - not the kind of art that has a moral, but purely a series of pleasurable encounters, sensual pastimes, and beautiful objects and sceneries; even people aren't people, they're just actors on life's stage (the main reason Dorian turns on Sibyl is her desire to stop acting, and her view that staged emotions are nothing to what people really feel). Everything 'unartistic' about life gets dumped onto a work of art, Dorian's portrait. This is a misuse of art, it seems; at the same time, the portrait does show truth as it isn't always perceived in day-to-day life, which is what good art can do.
Where does Lord Wotton enter into this? He leads Dorian astray (without, I think, realizing the extent to which he's done so), and while he's always full of witticisms and what seem to be incisive observations about life, by the end of the book you see that he doesn't understand Dorian and what's become of him. His understanding of himself is also limited. Take his views on women, for instance. He has much to say about women, and a good deal of it isn't flattering. But what's telling is that his observations of women really best apply to himself; what he says about them holds truest for his own character, way of thinking, and role in life. Wotton is full of words, and he talks as if there's always a bunch of dinner guests around him waiting to hear his next witty gem, but at the end of the day, there's little telling what's behind the words. Can the things Wotton says be taken at face value? Dorian unfortunately does, and you sense by the conclusion of the novel that Wotton has no depth.
Wilde had to revise the novel in the face of public outcry because of passages containing material deemed indecent. There's strong homoeroticism in the novel - in the way Basil regards Dorian possessively and worshipfully, and in the relationship between Lord Wotton and Dorian, with the older man holding out seductive promises to the younger one about a new way of life. Throughout the novel, Wilde explores the idea of people leading double lives... and how doing so may break them. Critics at the time made a number of negative comments about the book, that it would corrupt people who came into contact with it and that Wilde himself must be embracing hedonism. But did Wilde really support Dorian's hedonism? And in what ways can a book be 'immoral'? Simply for its content, or for how the artist works with that content and presents it?
I love how the book opens. Though the dialogue can sound unnatural, Wilde has a way with sensory details:
The studio was filled with the rich odor of roses, and when the light summer wind stirred amid the trees of the garden there came through the open door the heavy scent of the lilac, or the more delicate perfume of the pink-flowering thorn.
Ending like that, on 'thorn,' introduces an uneasy note to the heady sensuality and sets the tone for the rest of the book.
(I've read this for the Classics Club Challenge.)