The book is regarded as a quintessential example of "hard sci-fi", as its plot is guided by technology without any foray into the fantasy genre. It was nominated for the Hugo Award for Best Novel in 1971.
Tau Zero follows the crew of the starship Leonora Christine, a colonization vessel crewed by 25 men and 25 women. The ship is not capable of FTL travel and so is constrained by relativity. Its engines operate two modes, acceleration and deceleration. The deceleration module becomes damaged during the trip. Because the engines must be running at all times (to provide particle/radiation shielding), and because of the hard radiation produced by the engines, the crew can neither repair the decelerator nor turn off the accelerator. Instead, the ship must accelerate indefinitely, crowding closer and closer to the speed of light.
Much of the novel deals with the crewmembers' reactions to the knowledge that they are being carried further and further into the future (because of the extreme time dilation caused by their ever-increasing velocity) and away from any possibility of contact with humanity. The novel derives considerable power through descriptions of the changing and extreme time dilation effects.
The storyline is reminiscent of the long poem and later opera Aniara where the ship also was unable to stop and doomed to travel endlessly. Anderson's novel has a more upbeat ending, however: when all hope is gone, the crew finally find a drastic solution to their situation (albeit the solution does not conform to modern thinking on the evolution of the universe).
An element rather incidental to the main theme is the unique political situation in the Earth from which the protagonists set out: a future where the nations of the world entrusted Sweden with overseeing disarmament and found themselves living under the rule of the Swedish Empire. This sub-theme reflects the great interest which Poul, an American of Danish origin, took in Scandinavian history and culture. In later parts of the book, characters compare their desperate situation to that of semi-mythical characters of Scandinavian legend, with the relevant poetry (which Poul knew extremely well) extensively quoted.