The premise of this novel is weird and delightful. At a party in the year 2031, with the American civil war over and the Republic of California declared, it’s time to pop the corks in Los Angeles. Our protagonist, Ari, is slightly wary: she is newly out of prison and declares: ‘When I think about parties I think about bombs.’ This is the least of her worries, as in the inebriation she meets ‘the same woman she had seen earlier, wearing a silver pleated dress, but she’d changed into casual clothes now, jean shorts and a T-shirt’. Moreover, some people are calling her Ibari – ‘My legal name… I’ve literally never used it. It’s just the name on my ID.’ At the same party, the host disappears, and not just to stock up on the drinks.

I do not think it is too much of a spoiler to say that Exit Party is a Hugh Everett III-inspired quantum novel: it uses the many worlds theory. Others have enjoyed this idea, in particular Andrew Crumey (Möbius Dick is an underrated masterpiece); but it is probably most recognisable from the 1998 film Sliding Doors. Or indeed Jorge Luis Borges’s story ‘The Garden of Forking Paths’. What if every decision creates a new, branched universe?

The Ari we originally meet has just been serving 18 years for plotting a murder. Another Ari is a much more likeable individual – a philanthropist, not a terrorist. In the background of some universes is ‘Service’, a shadowy organisation of ‘dark cars moving down the block, men and women in forgettable clothing stopping our parents on the street for brief random interrogations’. As one character observes: ‘PTSD is a timeline corruption. The past interrupts the present.’ The novel’s central riddle is how different do you need to be to be different? In endless iterations, what is unchanging?

Emily St John Mandel is clearly having fun as she rollercoasters through various Americas – the one which is ‘the United State’; the one which has fragmented. She slightly indulgently allows some call-outs to her previous oeuvre, such as the lunar landscape of Sea of Tranquility. But these structural observations are nothing compared to the emotional kick of it all:

There’s a whole religion devoted to the idea, in the place that I’m from. The idea is that after you die, your neurons keep firing for a while. Something like seven minutes… some people think that when you die you get one last dream.

This dreamlike, sometimes nightmarish, often unsettling story would be a good one on which to say goodnight.