Transition, By Iain Banks

The absence of the middle initial M in Iain Banks' name on the front cover suggests that Transition is being sold by his publisher as one of the author's mainstream works, as opposed to his equally successful science-fiction books.

In fact, this 24th novel from the critically and commercially successful writer makes that distinction almost completely obsolete, its complex and wildly imaginative storyline ostensibly set on Earth, but infused with such mind-boggling phenomena as to make that setting seem stranger than any alien planet.

Banks begins in deliberately obfuscatory fashion, with half-a-dozen disparate narratives very gradually filling in the blanks in a very odd universe indeed. The crux of Transition is that the world we live in is one of an infinite range of parallel worlds, and that, for the best part of a millennium, certain individuals, called transitionaries, have been able to flit between physical bodies in these different worlds using a drug called septus.

These agents are under the control of the Concern, a secretive organisation that guides them to intervene in particular societies, executing pre-emptive strikes on people who would otherwise have gone on to have a very bad influence on that world.

The central narrative is delivered by Temudjin Oh, a skilled assassin carrying out the Concern's orders but beginning to worry about the moral consequences of his actions. Dancing around that story we also get the lives of Madame d'Ortolan, a powerful and possibly ill-intentioned member of the Concern's higher echelons; Mrs Mulverhill, a renegade agent bent on rebellion; the Philosopher, an enigmatic, state-sponsored torturer; Adrian Cubbish, a greedy and ambitious City trader; and Patient 8262, faking a mental disorder in a state institution to escape the world of transitionaries for a while.

Fans of Banks' previous work won't be surprised to hear that over the course of the book, these narratives snake around and through each other, gradually coalescing into a corker of a thriller, a classic good versus bad tale, and one which the author uses to tackle some seriously big moral and philosophical issues – but always in his typically light-handed and darkly humorous fashion.

As Temudjin Oh becomes increasingly doubtful about the work he's doing for the Concern, the real purpose of that organisation comes into question, and the emerging answers are sinister in the extreme, leading to a climax that has the fate of the entirety of humankind hanging in the balance.

As always with Banks, the imaginative detail is frequently stunning. By creating a universe of infinite different but related worlds, the writer has given his mind free rein to create and describe all sorts of weird and wonderful alternatives to our society. With the abilities he has given his transitionaries and other similarly gifted individuals, and the power they have bestowed on the Concern, he tackles the issues of the responsibility of power, the moral implications of intervention into other societies, and even the philosophical conundrum of what constitutes life itself.

All of which will be familiar to fans of his Culture sci-fi books, but which carry more gravitas here, being an integral part of the world we see around us. Despite their extraordinary powers, the likes of Temudjin Oh and Mrs Mulverhill seem all too human, lending vital reader empathy to the nerve-shredding climax in the streets of a Venice that may or may not be in this world.

Transition is a book that makes you think, one that makes you look at the world around you in a different light, and it's also a properly thrilling read. If only more contemporary fiction was like it.