Professor Caritat (named after the surname of the Optimistic philosopher better known to us as the Marquis de Condorcet), a specialist on the thinkers of the Enlightenment, is a cross between a sophisticated Candide and a Gulliver. He is a citizen of Militaria, a repressive state run by a military dictatorship. He has to flee from that and embarks on a mission to find a state in which the ideas of 18th century philosophers have been applied and to see how these ideas have turned out in practice. He is looking for the best of all possible states. But in each of these countries he gets into trouble, wittily described, and has to move on.
The first country he visits is Utilitaria, which is governed by the ideas of Jeremy Bentham; and a soulless place it is, where there is no place for anything that cannot be proved to be `useful' to its society.
The next country is Communitaria. Its government is committed to total respect and equal treatment for every ethnic and every religious community, but it interprets respect in such a way that any comment which might suggest that one way of living or one set of ideas is superior to another, and every criticism of any group, is severely punished, so it is in effect an extremely intolerant state.
From there Caritat escapes to Proletaria - named, he is told, after the class that had brought it into being, although that class, like all others, has since withered away, as indeed has the state itself.
This turns out to be a phantasmagoria, from which he awakes to find himself in yet another country, Libertaria. Here free enterprise is rampant, every public service is in the process of being privatized, financial extortion is the name of the game, and woe betide those who can't play it. The country seems to be an ally of Militaria: though Libertarian `freedom' is missing there, Militaria's maintenance of `order' is something the Libertarians admire.
Finally, on his way out of that country, the Professor makes it to Minerva, a border town in the North of Libertaria. There he comes to the wise conclusion that what was the matter with all the dystopian states he had visited was that the single-minded pursuit of just one desirable aim (Order, Welfare, Respect, Equality, Freedom of Action) leads to the suppression of all the others. Condorcet had observed that all human ideals are linked together in an indissoluble chain. More practically, Isaiah Berlin (whom Lukes does not name in the text, though he does mention him in the bibliography) has taught that there needs to be a trade-off between all these desirable aims. How to strike this balance must be a never-ending quest, requiring much Wisdom.
One of his interlocutors in Libertaria had told him that people once believed that there was a state called Egalitaria north of their country, but that they had found that it did not exist and was in fact a utopia. Perhaps that is the reason why the book ends in Minerva, with Caritat seeing that just beyond the border is a crossroads from which several roads extended.