John Bayley, in his introduction to my Penguin Classics edition, describes the novel acutely as "the most sensational combination of a stage drama, indeed a melodrama, with a lengthy and elaborate novel, unfolded with the greatest delicacy and sophistication." He quotes James from his Notebooks, "The divine principle is a key that, working in the same general way, fits the complicated chambers of both the narrative and the dramatic lock."
The plot is simple: Kate Croy, poor and dependent on her aunt, Mrs Lowder, manipulates her lover Merton Densher to marry the American heiress Milly Theale, fabulously rich, young, and dying. The drama lies in the twists and turns of the conspiracy and its unexpected outcome. More significantly, the novel dramatizes Merton's gradual understanding of Kate's plan, and the effect of that understanding on their love. Bayley aptly compares them to Macbeth and his wife, united in their conspiracy to kill the king, only to be undone by their success.
Like a great play, the novel has some unforgettable scenes. It opens with Kate pleading to be taken in by her useless father, who turns her down, and advises her, instead, to return to her rich aunt, so that he can collect on Kate when she is matched with some wealthy suitor. The horror here is Dickensian, but James transmutes Dickens into a finer key. Another poignant scene depicts the lovers' tryst between Kate and Merton before the corruption sets in. The depiction of love is entirely believable, perhaps, a result of James's discovery of love for some young men who entered late in his lonely life, as Bayley suggests. In this aspect, and others, the novel is a clear advance on The Portrait of a Lady, in which Isobel Archer marries Gilbert Osmond not because she loves him, but because it is the right thing to do.
Some crucial scenes take place off-stage, and they appear through dialogue and meditation. Deduced through social interaction and debated by individual consciousness, the events are rendered deeply ambiguous. What is Milly's motive in receiving Merton? We cannot be sure. For a long time we are not even certain how ill Milly is, the same uncertainty that besets her predators, as it does in normal social intercourse in real life. The novel immerses us in the indirections, hesitations, complacencies of talk and thought; it refuses to tell us what to think. Here lies its immense challenge for the modern reader, and its immense aesthetic achievement.