On a world in a different universe to our own, rival kingdoms battle for supremacy, but a deadly plague, the pterthacosisis, is wiping out the population. The only hope of survival is for the rapidly diminishing group of unaffected to flee and start fresh. But the only potential place for sanctuary is an unexplored sister planet, and it's thousands of miles away across space and seemingly out of reach for a people whose technology is broadly at the stage earth's was in the 17th century. Then a scientist has the great idea to build a giant airship and sail there...
All Bob Shaw's strengths are displayed here: an ingenious and innovative story line, believable characters who you can care about, and gleefully implausible plot twists that you can happily accept without too much worry. In this case the implausibility comes from the fact that space travel between planets is possible by balloon. This unusual premise is unlike most other Bob Shaw plot twists in which the scientific explanation usually only goes to add weight to just how implausible the idea is. No explanation is, very wisely, offered other than a reference to pi having a value of 3 in this universe.
With no time wasted on the science, more time is left for the rip-roaring adventure and that makes this yarn entertaining and one that is well worth reading. Whether the whole trilogy is worth reading is debatable as towards the end of the second book Shaw starts to lose interest in the tale, and he definitely pushes the idea too far in the third book. A short balloon trip through space is just about acceptable if you're willing to suspend your disbelief, but regular interplanetary travel and warfare in wooden spaceships takes some swallowing.