|Cover artist||Bob Eggleton|
|3 December 1993|
|Preceded by||Timelike Infinity|
Dura and her fellow human beings live inside a neutron star, as they have for generations. As long as they can remember, "glitches" (instabilities of the magnetic field inside the star caused by changes in the star's rotation) have happened from time to time. As the novel starts, the worst glitch that anyone can remember threatens to destroy her home. To survive, Dura must travel to a far-off city, and eventually outside of the star itself.
Flux is so called as the main race of the book, microscopic human-like beings who live within a neutron star, travel around using the magnetic field lines within that star. It also relates to a bigger element of the plot, dealing with instabilities in this field which affect many characters in different ways.
As the novel begins, a glitch — an instability of the magnetic field inside the star caused by changes in the star's rotation — is about to destroy a net made of ropes, where a group of 50 humans live. During this several of the older humans are killed, and importantly the humans lose their main food source, a herd of "air pigs", animals indigenous to the star.
To find more food, Dura, together with her young brother Farr, Adda (the eldest of the humans and one of the novel's main characters), Philas (wife of a man killed during the glitch; this man was also seeing Dura), and six other adults travel high into the top of the mantle of the star to find food in the forest.
While there Adda is injured by a pregnant air pig. Soon the humans encounter Toba Mixxax, another human from Parz City. Parz is a massive wooden city where other star-humans live, with a functioning economy and upper and nether classes. As becomes apparent, the ancestors of Dura's group did originally live in Parz, but left when their belief that the Xeelee should ultimately be accepted as being for the good of humanity was not accepted by the rulers of Parz. A hospital, "The Hospital of the Common Good" in the heart of Parz City, is Adda's only chance for survival.
While in Parz, we meet several other characters, including Muub, the head physician and advisor to Hork, the administrator of Parz. To pay for Adda's treatment, Dura's labour is sold to a mantle wheat farm, and Farr is sold to work in the underbelly of the city as a miner.
Farr makes two friends while here: Toba Mixxax's son Criss, and Byza, a fellow miner. Criss teaches Farr to board (using a specially constructed plate to "surf" along the flux lines), an ability which allows him to escape from the eventual attack by the Xeelee.
After various plot points, the characters realise the instabilities are actually being caused by the attack of the Xeelee, and the next instability could destroy both Parz and possibly the star itself. Hork calls for Muub, Dura (called due to her experience as a star-human), Adda, Farr and a scientist to go down into the uninhabitable centre of the star in a specially constructed vessel and try and retrieve ancient weapons, supposedly left by the humans who originally created the star-human race. When they get there however they discover that the neutron star had been turned into a missile aimed at The Ring that is being constructed by the Xeelee and so the Xeelee have been attacking it to stop it. They also discover that they were created by normal humans to ensure that the star remains on course but that would result in the destruction of the star and themselves. They decide to alter the course of the star so that it misses The Ring and so the Xeelee stop their attacks.