Set partly in a future surveillance society, partly in ancient Carthage and 1970s Ethiopia, partly in contemporary Greece and London and partly in the synaptic passageways of the human brain, this huge sci-fi detective novel of ideas is so eccentric, so audaciously plotted and so completely labyrinthine and bizarre that I had to put it aside more than once to emit Keanu-like ‘Whoahs’ of appreciation.

Science fiction in general is having an interesting moment right now, as writers and filmmakers respond to the loopily futuristic contemporaneities of robotics and AI research, but Nick Harkaway goes further than most in this vast and baroque novel. It’s a technological shaggy-dog tale that threatens to out-Gibson William Gibson, a dense and angry fable about political coercion and control, and a loopy, self-swallowing story about storytelling. It is huge fun. And it will melt your brain.

Where to start? Gnomon opens with Mielikki Neith, ‘Inspector of the Witness’, presiding over an inquiry into the death of one of London’s citizens. In this distantly recognisable futuretopia — whether it’s u- or dys- is a matter of opinion, at least to start with — all life is lived under the System, an ‘ongoing plebiscite-society’ in which citizens contribute by frequent voting to the maintenance of an equitable and just polis. Overseeing the System is the Witness, a benevolent AI that has unrestricted access to a limitless network of public and private surveillance. Sure, it sometimes has to perform a bit of open-brain surgery to make sure people aren’t conspiring against their peers, but don’t worry: those with nothing to hide have nothing to fear. ‘Justice has been perfected, and the Witness is everywhere.’ What could possibly go wrong?

Naturally something has. An elderly woman called Diana Hunter, once a writer of ‘obscurantist magical-realist novels’, has died on the table while being quizzed by the Witness. As the investigator supervising the enquiry, Mielikki Neith is tasked with going over the memories that surfaced during her interrogation. So begins an Inception-like sequence of nested realities, as Neith bumps up against the dead woman’s ‘narrative blockade’: a Scheherazade strategy to distract and bedevil an invader with a protective layer of stories.

Soon Neith is zooming through a cosmos of different lives: a female alchemist performing a magical ritual in the ancient world; a financial whiz kid haunted in contemporary London by a shark made of computer code; an Ethiopian artist who can walk through walls; and the fearsome exploits of Gnomon, a data assassin from the future commissioned by a sentient planet to kill the universe. All these stories are linked; the Inspector’s exquisitely difficult task is merely to work out how.

This is a great wriggling plot, full of red herrings, techie sidetracks, political invective and dead ends, but it turns out to be startlingly elegant in motion. Previous books such as Angelmaker and Tigerman have shown how good Harkaway is at packing hurtling genre narratives with shrewd and reflective prose, but in this one he’s working at full stretch: 700-odd pages power relentlessly by, only to touch down with the delicacy of a SpaceX rocket on — ah yes — the only possible ending. Whoah, indeed. I wanted to give it a round of applause.