The novel pivots on the figure of Prince Myshkin, who returns to Russia after an extended stay in Switzerland, where he was treated for "idiocy"—that is, mental illness.
The protagonist’s trajectory unfolds within a setting crowded with characters, from the somber Rogozhin (virtually the personification of evil) to a succession of opportunists and nihilists, through to two female characters (Nastasya and Aglaya) who, albeit in different ways, vie for Myshkin’s love.
The clash between the two women, Nastasya’s flight from Myshkin (who intended to marry her out of pity rather than love), and Rogozhin’s murder of Nastasya ultimately drive Myshkin back into madness.
The novel’s central theme is Myshkin’s innocence within a false, devious, and ambiguous world that the protagonist does not comprehend, and whose conventions and rules he tends rather to disrupt through spontaneous and sincere behavior.
Myshkin constitutes a disruptive element to which society fails to respond, bewildered and taken aback as it is.
The dilemma is the one voiced by Aglaya’s parents:

There was something strange about the matter, and just how strange, exactly? Or was there, rather, nothing strange at all?

.
Naturally, the parents seek an answer in the opinion of society and its most prominent figures, arranging a reception at which Myshkin will astonish (and partly mesmerize) everyone with his disconcerting behavior.
In some ways, Myshkin recalls the figure of Christ in the famous encounter with the Grand Inquisitor in The Brothers Karamazov: sincerity, simplicity, a sense of pity, and the will to bestow love are all elements alien to our society, and somehow misunderstood or rejected.
Madness is thus not a negative condition, but a different—and truer—vantage point on reality and human relations.
The characters seem somehow to grasp Myshkin’s power; they are drawn to it, yet dread it.
Nastasya understands him better than the others, yet shrinks from him for fear of hurting him; Aglaya loves him, yet wishes he were different, more compliant with the rules of society; and Rogozhin hates him precisely because he perceives that Nastasya is irreversibly drawn to Myshkin’s nature.
The other characters too—and they are countless—act out of the same ambivalent sentiment, wavering between contempt and admiration.
Alongside this central theme run innumerable other lines of inquiry, some characteristic of contemporary society (such as the nihilism already explored in Demons), others more universal: among the most compelling are the anticipation of death and the anguish that grips humankind, the face as a mirror of the soul and of the observer, and the theme of evil and hatred, embodied by Rogozhin and evoked within the gloomy atmosphere of his house.
The Idiot is a singular novel.
It is highly traditional in its narrative style, which is so overburdened with dialogue as to almost resemble a play, yet it is profoundly innovative in terms of the themes it addresses.
The near-total absence of the religious theme (dominant in The Brothers Karamazov) allows for the explosion of other concerns closer to an existential dimension, in which the individual must seek selfhood autonomously rather than relying on divinity or metaphysics.
Even more than his other novels, The Idiot gives expression to the "new man" and thereby opens the door to postmodernism.
The narrative pacing is not consistently compelling (closer to Demons than to Crime and Punishment or The Brothers Karamazov), and in strictly literary terms, it is not particularly successful.