Duncan Makenzie is the latest generation of the 'first family' of Titan, a colonised moon of Saturn. Originally settled by his grandfather Malcolm Makenzie in the early 23rd century, Titan's economy has flourished based on the harvest and sale of hydrogen mined from the atmosphere, which is used to fuel the fusion engines of interplanetary spacecraft.

As the plot opens in 2276, a number of factors are combining to make a diplomatic visit to the 'mother world' of Earth a necessity. Firstly, the forthcoming 500th anniversary of US Independence, which is bringing in colonists from the entire Solar System, obviously needs a suitable representative from Titan. Secondly, the Makenzie family carry a fatal damaged gene that means any normal continuation of the family line is impossible—so both Duncan and his "father" Colin are clones of his "grandfather" Malcolm. Human cloning is a mature technology but is even at this time ethically controversial. And thirdly, technological advances in spacecraft drive systems — specifically the 'asymptotic drive' which improves the specific impulse and thrust by orders of magnitude — means that Titan's whole economy is under threat as the demand for hydrogen is about to collapse.

The human aspects of the tale center mainly on the intense infatuation (largely unrequited but not unconsummated) that the two main male characters, Duncan and Karl Helmer, develop for the vividly characterized Catherine Linden Ellerman (Calindy), a visitor to Titan from Earth in their youth, and its lifelong consequences.

A number of other sub-plots suggest some sort of greater mystery, but remain unexplored. The book ends with him returning home with his new "child" Malcolm (who is a clone of his dead friend Karl), leaving the other plot threads dangling.

The book is the first work of science fiction to feature a starship powered by a black hole