The book is a quintessential example of "hard sci-fi", as its plot is dominated by futuristic technology grounded in real physics principles.

The text consists of narrative prose interspersed with paragraphs in which Anderson explains the scientific basis of relativity, time dilation, the ship's mechanics and details of the cosmos outside.

As there is no hope of completing the original mission, the crew increase acceleration even more; they need to leave the Milky Way altogether in order to reach a region where the local gas density, and the concomitant radiation hazard, are low enough that they can repair the decelerator.

The ship's ever-increasing velocity brings the time dilation to extreme levels and takes the crew further and further away from any possibility of contact with humanity.

However, they find the vacuum of intergalactic space insufficient for safety; they must instead travel to a region between superclusters of galaxies to make repairs.

They must wait, flying free but essentially without the ability to change course, until they randomly encounter enough galactic matter to decelerate adequately to search for habitable planets.

Throughout the story, Charles Reymont, the ship's Constable, fights to keep hope alive in the confined community and at the same time maintain order and discipline, sometimes at great emotional cost to himself.

He explains his system to his partner Chi-Yuen Ai-Ling: The human animal wants a father-mother image but, at the same time, resents being disciplined.

Therefore she adds benignity to the attributes of Ultimate Authority.The storyline is similar to that of the long poem and later opera Aniara, in which the ship was unable to stop and doomed to travel endlessly, but Tau Zero has a more upbeat ending.

By the time the ship is repaired, tau has decreased to less than a billionth and the crew experience "billion-year cycles which pass as moments".

The universe collapses (a process the starship survives because there is still enough uncondensed hydrogen for maneuvering outside the growing singularity) and then explodes in a new Big Bang.

The voyagers then decelerate and finally disembark at a planet with a habitat suitably similar to Earth, on which the vegetation has a vivid bluish-green color.

In the final, ending parts, much of the novel deals with the crewmembers' reactions to being the last remnants of humanity, and the prospect of being confined with their colleagues indefinitely.

Though they were prepared to "lose" twenty Earth years during their journey and spend five on board the ship, the knowledge that they are being carried further and further into the future has various effects on the psychology of the characters.

Incidental to the main themes is the political situation on 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.