“Inter faeces et urinam nascimur,” St Augustine of Hippo is credited with remarking. “We are born between shit and piss.” But in most societies, most of the time, we strive to keep those things out of sight and out of mind: there are deep taboos settled on these most basic human commonalities. Why might that be?

Sam Byers’ third novel poses the question in earnest. Not since Timothy Mo’s Brownout on Breadfruit Boulevard, I think, has there been a mainstream literary novel so fiercely and lovingly committed to the feculent: whole paragraphs and pages are dedicated to mucous, vomit, slicks of warm diarrhoea, puddles of piss, maggoty sores and liquefying rotten meat. You may find that disgusting. There again, you may be part of the problem. But I’m getting ahead of myself.

Byers’ protagonist, Maya, has been homeless for a year, and is living in a squatter camp when she’s picked up by the police in a raid. We don’t learn too much about her background – only that she had a white-collar job and loving parents before suffering an existential crisis (something to do with living under capitalism, and the intolerable expectation of others that she be happy) and steadily withdrawing from her own life. Anyway, because she scrubs up well, seems essentially sane and isn’t a hopeless addict, capitalism wants her back.

While she’s in custody, a couple of unctuous tech company PR men show up to offer her a deal. She gets a flat, a phone, a laptop and a job; and in exchange she’s to document her new life in an Instagram feed called “Maya’s journey”. The job is at a company where she helps to filter “inappropriate” material from the internet. Byers’ humour, lower in the mix than in his previous novels Idiopathy and Perfidious Albion, comes out here: she spends every working day watching a stream of vile images and swiping left or right to accept or reject them, like someone Tinder-dating the dark web.

By way of therapy or corporate modishness, her employers send workers on periodic wellness retreats. On returning from one of these – full of kale smoothie and whalesong, and craving cheap white bread and chocolate – Maya goes on a binge, whereupon a long-delayed and very substantial poo sparks an epiphany:

What I’d produced was glorious. My shit was so abundant, so strong, that it formed a kind of cathedral or castle, a deep-brown edifice that rose out of its rust-coloured moat and tapered to a spire about halfway up the bowl.

At last, Maya feels alive. And when she then makes herself spectacularly ill by dipping a bit of bread in her own shit and eating it, she feels still more alive. Late capitalism sent her on a “wellness retreat”; but it’s on her own initiative that she burns her bridges and heads off on an illness retreat. She falls in with an eccentric and equally marginalised woman called Zelma, who likes to annotate glossy magazines and deface wellness-themed billboards; they squat in an industrial estate and, well, roll around in their own filth. Soon – “Come join our disease!” – they attract a handful of acolytes.

An enthusiastic embrace of everything that normal society tends to disapprove of (crapping where you eat, welcoming lesions and pustules, wearing necklaces of dead rats, and – worse – doing nothing obviously economically productive) becomes the substance, as they see it, of a connection with a more essential truth and a deeper freedom; as well as being a surprise hit on Instagram. This is what you might have got if Melville’s Bartleby had gone the dirty protest route.

The question that the novel chews over, or at least makes available for consideration, is whether Maya and her friends’ behaviour is a form of radical anti-capitalist resistance, a deeper sort of existential heroism – or whether, as “society” predictably enough seems to think, it’s simply a manifestation of mental illness. Those things may not be mutually exclusive, of course; nor may the answer be the same for each of the women who joins the group – and it’s one of the novel’s strengths that it allows for a complicated response.

You sometimes see it said of high-concept works like this one that the idea is ambitious but the execution falls short. Here, I’d put it the other way round. Byers is a keen and effective stylist, and he’s superbly astute about the complex shifts and negotiations between his characters. But the big ideas here – the repressions and paradoxes of “civilisation”, the loops of resistance and co-option and commoditisation – are so big and clunking (and well established enough in the greatest hits of Freud, Foucault, Michel de Certeau, JG Ballard, John Gray, Sartre …) that they overshadow the subtler work that Byers’ prose is doing. For amid the lurid colour and the grand theoretical gestures is a poignant story of friendship and isolation, of human connections made and lost. There’s something interesting growing in all that filth.

• Come Join Our Disease is published by Faber (£16.99). To order a copy go to guardianbookshop.com. Delivery charges may apply.