The coolest contemporary movie star, Keanu Reeves, added to his portfolio in 2021 by creating a comic book series called BRZRKR, co-written by Reeves with Matt Kindt and illustrated by Ron Garney. The title, a vowelless “berserker”, references those Viking warriors who fought with trance-like fury. In the comic, this is “B”, also known as Unute, an immortal who looks very much like Keanu Reeves (Reeves has signed up to play the role in the forthcoming Netflix adaptation). B goes from prehistory to the present day via a series of extremely violent fights, dismemberments and killings. He can be injured, but not killed: his wounds heal, missing body parts grow back, and on such occasions as his body is entirely annihilated a giant magic egg appears out of which he later emerges.

BRZRKR, as title, suggests a word too muscular for piddling little lower-case letters, and frankly too macho for vowels. Grunt, shoot, stab, kill, rip bodies apart with bare hands, is the whole game here. And if “BRZRKR” also looks somewhat like a typographical representation of somebody blowing a raspberry, there is something ridiculous about the whole thing, too. There is little by way of actual story. The fundamental samey-ness of the conceit, with diminishing returns of ripped-out intestines and gore, knives, arrows, bullets and blood spray, flattens and banalises the telling. Still, it has Keanu as main character, and Keanu is cool.

Now Reeves has expanded the franchise into a novel by collaborating with British author China Miéville – I’d call Miéville “the coolest contemporary writer of the fantastic”, though the bar is rather lower than in movie stardom – with a title that is, at least, fully supplied with vowels. It’s Miéville’s first novel for 12 years. His previous books, from the new weird Perdido Street Station (2000) through the much-praised The City & the City (2009) to Embassytown (2011), dominated SF for a decade. His return is much anticipated by his fans.

The Book of Elsewhere revisits the material from the comics, fleshing things out, as a novel can, giving a more thoroughgoing and detailed account of the backstory. The main motor of the plot is B’s yearning for mortality. This is not a simple desire to die but a more nuanced intent to stop being immortal. The idea here is that it is our mortality, our living-towards-death, that gives life meaning and richness, and B wants that. But no matter what is tried, he cannot get past his unkillability.

Where did B come from? His mother had a painful-sounding encounter with a supernatural being, perhaps a god. Struck by a bolt of blue lightning in a sensitive area, she becomes pregnant with Unute. He learns his origin as he grows up.

“So my father is not my father?” Young B asks his mother.

“Hush, silly,” she replies. “Your father is your father, he’s your dayfather and the blue lightning is your nightfather.”

B roams through prehistory, righting wrongs, fighting and killing bad guys, then does the same thing through history, like Christopher Lambert’s Highlander, except that decapitation would not slow him down. In the present day we find him part of a US military unit that deploys him on various black ops missions. A scientific branch of this unit, headed by Dr Diana Ahuja, is also studying B’s unusual powers. This research has produced various technological and military advances (a new type of helicopter is described as “a spin-off technology”, which may be a joke), but nothing to solve B’s fundamental immortality problem.

There’s also a prehistoric pig whose mother was likewise struck by the magic lightning, and which spends immortal aeons repeatedly tracking B down in order to gore him with its huge tusks. Why the pig is so irked at B isn’t entirely clear, and there’s an off-kilter goofiness to this element of the story. B thinks if he can find a way to kill the pig then he can unlock the mystery of his own immortality, and so he brings it to the institute for further study.

“‘This pig … ’ Diana whispered. ‘This is farce. The repetition of you, the original tragedy.’” It’s not every superhero story that includes references to Marx’s Eighteenth Brumaire, but Miéville is a scholar of Marxism as well as a writer of fantasy.

Reeves has made clear that it was Miéville who wrote the novel, but it has to be said that The Book of Elsewhere is unlikely to take its place among his masterpieces. There is the daftness of the premise, the bittiness and repetitiveness of the narrative, the need to revert to scenes of tiresomely extreme ultraviolence.

There are, though, nice touches. At one point, Diana and B discuss establishing an objective scale for hatred.

“What’s the most universally hated thing in the world?” she said. “Child molesters? Hitler?”

“Not Hitler, unfortunately.” They were both silent awhile. “Mosquitoes,” he said.

“OK,” she said. “That’s good: they’re small, so they’re good for units. So, let’s say the hate aimed at one member of the Culicidae family measures one, I don’t know, culicid. A cull! Which means,” she said, “that if you hate something as much as you hate 10 mosquitoes, your hate is 10 culls. A decacull. That’s, say, dog shit on my shoe. Now, the Westboro Baptist church, say, I probably hate … ” She shrugged. “A good seven or eight kiloculls.”

But Miéville’s supple, inventive imagination gets stretched thin on the rack of Reeves’s original idea. Enter the Miétrix, but be prepared to be underwhelmed.

• The Book of Elsewhere by Keanu Reeves and China Miéville is published by Del Rey (£22). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.