The novel is built upon the inner conflict of the protagonist of first story. Adam Ewing, an American solicitor on a business trip through the South Pacific becomes directly involved in the conflicts between people from primitive and modern societies. The other five stories build on Ewing's reflections, interpolating the logical conclusions of these conflicts, leading finally to his own conclusions about the role his life plays in the future of humanity.
Adam Ewing inhabits the most historically informed of the six nested short stories: the author has devoted significant attention here to historical accuracy. There really was a tribe in the Chatham Islands called the Moriori who were conquered by the Maori just as Ewing witnesses. Ewing's narrative style is credibly of the period; it reads similarly enough to American authors of the period such as Edgar Alan Poe and Emily Dickinson. (That the author David Mitchell is British hasn't impeded his ability to craft American voices.)
The subsequent stories are visitations at times further and further into the future: the life of a young composer in 1931, an investigative reporter in 1975, a vanity publisher of our own time; a human clone in a future civilization on the brink of collapse, and finally a survivor of the final collapse. With the exception of the reporter's story, presented as a pot boiler mystery, these others also have an epistolary narrative style like Ewing's chapters. Each inhabitant of her time reveals certain aspects of the state of her world.
Each story demonstrates the hubris of those who seek power over others and the doom of all humanity when we all accept social stratification as the "natural order". The theme of heirarchy is echoed in the novel's structure; readers and critics are forced to acknolwedge its existence. Science and history show that such inequality between people is decidely unnatural, as does this novel.
I assert that it is entirely Adam Ewing's story, including the parts projected into the future(s). As Ewing comes to the end of his own adventure, it is as if he had peered into the future and had seen the composer, the reporter, the clone, and the aftermath survivor. Of course he is (or thinks he is) deciding on a particular new course in his life based on his own experiences. But because we have seen humanity's future history we know his conclusions are correct. Ewing vows to fight the so-called natural order, even though his lot is favorable in it, because he fears it will be humanity's downfall; having seen the future for ourselves, we know he's right.
There is much more a critic can say about Cloud Atlas. Like Bob Dylan, for example, David Mitchell has filled his prose with potentially symbolic language without always driving home the point. There is plenty more to say (as many critics will have done by now) about determinism and materialism. There is a film now produced by the Wachowski film draws further meaning from it some of which otherwise isn't explicit in the novel yet illuminates the original text. I present a certain subjective view here, similarly, and don't mean to say my view holds water in every other light but this is the meaning I personally drew from a casual reading.