You know something's not normal as soon as you start the first chapter of this strange novel. The people in it are human -- it says so, right there on the page -- but you soon realise they're not the same kind of humans you meet on twenty-first century Earth.
Certainly they seem to have the same feelings, desires, hopes and fears -- the same character -- as present-day people, but they inhabit a world physically far different from our own.
Baxter's achievement in Flux is to have created a believable world out of a single extrapolated idea so bizarre that a lesser (or in SF terms, 'softer') novelist would have balked at: his characters inhabit the mantle of a star. They are genetically re-engineered, microscopic humans, designed to suit their almost unimaginable environment. It is to Baxter's credit that we do imagine it, even though the novel does, at times, have the feel of a medieval fantasy. Baxter's aliens, the Xeelee, also feature in the background of this story.
The physics and biology appear to be well worked out from the initial science-fictional extrapolation, and readers who like their SF hard will have fun analysing Baxter's world-building.
The story is mainly that of Dura, a young woman who scrapes a living in the 'upflux' -- a kind of scavenging area away from civilisation, crossed by lines of concentrated magnetism. One of the periodic 'glitches' in the 'magfield' leaves her homeless, and she and the few other survivors of the cataclysm have to find a way to live. A group of them decide to head for Parz City -- a place they've heard about but never visited.
Dura and her younger brother have adventures on the way, as well as at the city itself -- an enormous wooden structure suspended at the pole of the star. We also see something of the life of the city-dwellers, and learn of the city's ultimate purpose.
Baxter's writing style is robust but smooth, not unlike a less-honed version of Arthur C. Clarke, and is thus eminently readable. Although Flux is a story of characters in a unique but consistently imagined world, the events that occur are derived directly from that strange world's peculiarities. This is hard SF of a rare kind.