Adam Roberts specialises in hard science fiction with a heart, and his books more often than not find ordinary people placed in tricky situations in worlds where things are turned on their head, often just by a tiny tweak of technology or history. Purgatory Mount is two books in one, encompassing all of Roberts’ familiar themes, but slightly missing the landing in knitting the two together.

The book starts with the crew of an interstellar spaceship preparing to land on a strange planet, the striking feature of which is a huge, apparently man-made mountain, the peak of which escapes the atmosphere of the planet into space. The crew, given human-appropriating titles and names to emphasise their alien nature, are tasked with investigating the mystery of the mountain before others from their home planet follow on and discover it themselves.

From this hard sci-fi opening, the book takes a sharp turn into an apocalyptic near future America, riven by civil war and under the heels of a totalitarian government. Seemingly unconnected to the first chapter, Roberts lets the story unfold in the pleasant company of Otty, a young hacker caught up in something much bigger which affects her whole world.

“The United States of Amnesia”, as this section of the book is titled, is an engaging thrill-ride, creating perilous situations for its very likeable characters to try to survive through, and if Purgatory Mount had just contained these chapters it would make for a fine book in its own right. So, too, would the other half of the story, but it doesn’t quite feel as though they are taking place in the same world.

In his afterword, Roberts fears the link between the strands may have been too obvious, and your mileage will vary on how accurate his assessment might be. Regardless of this, Purgatory Mount is half a great story, half a really good one, and it doesn’t really matter if the author intends them to be read together, because it all adds up to another fine book from one of modern science fiction’s premier talents.