The book begins with Kundera explaining Nietzsche's idea of the eternal return (and Kundera is incredibly well read, citing not just philosophy but religion and mythology as well): the only way that anything can have any meaning is if it can be repeated. Since this life cannot be repeated, the question becomes whether or not this life can have any meaning? Kundera does not give an answer to that question, choosing to instead flesh out his characters by not only giving philosophical weight - no pun intended - to the narrative, but by giving psychological insights on their motives and actions.
It is hard to say what the real narrative of the book is. Is it the meaninglessness with which Tomas engages in his endless womanizing? Is it the utter falsehood that Tomas tries to make real in claiming that sex has nothing to do with love? Despite his telling his wife that his womanizing has no effect on his love for her, it could easily be argued that what makes his promiscuity so depthless is the fact that he has no love for anyone. In the end, we see that the body and the soul are intimately connected, not divorced from one another. The interweaving of these multiple narratives is part of what makes the book so insightful.
The book would be little more than a trashy (at points) psychological romance novel, however, if it weren't set in the Communist invasion of Czechoslovakia; indeed, it is the psychological and philosophical insights that are born of the experience of Communism that give the book its weight. Communism's utopian ideology was so fanatical as to be murderous - similarities between this and violent religious fundamentalists today beg for comparison - and was utterly indifferent to the particular people in the particular countries that it imposed itself upon. Communism is the socio-political embodiment of the indifference of someone such as Tomas and as such is horrifically insightful.
The unbearable lightness of being is heavy; this is not a bad pun, but a recognition of the tremendous irony of it all. The isolation and erasure of meaningful, particular histories in a world devoid of giving and listening renders everything unbearably light: and therefore unavoidably and oppressively heavy. Substantive notions such as love lose all of their substance; to borrow from Marx, "all that is solid melts into air."
Kundera's book is both well written and incredibly tragic. Ideology at the expense of humanity - either your own or another's (or both!) - is at the root of any soul/body dualism: the infidelity between lovers or the totalitarianism of Communism (and isn't Communism simply soul/body dualism politically imposed on a mass scale?). Such soul/body dualism is nothing more than the imaginings of philosophers; the unbearable lightness of being cannot help but to become indifferent, oppressive and violent.
The lightness of being is at the focal point of this novel. We are first presented with a brief essay-like section on whether life is a light or a heavy experience, and if it differs for others. He uses historical figures as evidence as to the weight of life, some, like Parmenides, consider life to be a light burden, as 'lightness' is positive and so is being alive, and Nietzsche appears to agree, though for very different reasons. Throughout the novel, Kundera takes over the narration to discuss, contrast and compare the actions of the characters as regards to the philosophy of great men, trying to determine whether life is in fact light or heavy. The conclusion he seems to come to is that it is up to the person themselves to decide, and after that, to decide which out of light or heavy is the negative aspect.
In terms of story, most of the activity centres around Tomas and Tereze, who met through a bizarre sequence of activities. In another show of polarity, Tereze considers these amazing coincidences proof that they should be together forever, whereas as Tomas thinks it means that their relationship will be as fleeting and ephemeral as the chance of them ever meeting. We also get to see on of Tomas' (many) mistresses, Sabine, although the details of her life are presented more to understand Tomas.
About halfway through the novel, we are taken on an excursion into the way life was in Czechoslovakia, with the threat of the Russians and communism, and the way people were deluded. This part is interesting from a historical and social aspect, as our heroes are involved in the proceedings, but thankfully the author does not let his own political ideology take over the narrative at the expense of the characters.
Since we are being told this story by Kundera and not some nameless, faceless narrator, the writing is very playful, tangential and casual. Many things are explained then further explained in brackets (like so), which might seem like the author is bashing our head with the point he is trying to make, but it never comes across as this. Rather, we are thankful for such personal insight.
The book can be very sad, and very weighty, but most of the time it remains light-weight and playful - thus mimicking the subject matter and narrative structure of the story itself. The insight into the Czechoslovakia as a nation and as people is quite interesting also, but as said above, the focus remains on Tomas and Tereze's relationship, and through that, an analysis of all relationship's is made.
Yes, there is a wonderfully layered and textured story here. And for most writers and readers that would be enough. But the story is certainly secondary here to the powerful message that Kundera delivers about what it means to be human. In the first few pages he cuts to the core of man's existence in a complex, unpredictable world. I grabbed my highlighter after just seven pages, having already come across three basic Truths - yes, with a capital `T' - that I wanted to come back and revisit. "What can life be worth if the first rehearsal for life is life itself?" Therein lies one of the most complex questions ever asked. And in this book lies one of the most eloquent answers ever given.