Mordew ain’t the kind of place to raise your kids, as Elton John nearly sang. If they escape the ravages of lung worm, then they could stray into the Living Mud — a foul, oozing substance that spawns barely animate beings called ‘dead-life’. That’s if they avoid being packed off to serve the Master, the city’s Grand Inquisitor, who broods in his Manse, demanding regular tributes of children, his magic twitching throughout Mordew.

The world of Alex Pheby’s fourth novel is dizzying. But stick with it, and his splicing of Dickensian social satire and rackety, steampunk fantasy is beguiling. The titular city is exuberantly realised, the sort of setting H.P. Lovecraft and Hogarth might cook up on a mescaline binge.

Our young hero is Nathan Treeves, a scamp from the wrong side of the Glass Road, the dividing line that separates the city’s haves and have-nots. His father is mortally ill; his mother entertains ‘gentleman callers’ to keep their heads above water. But Nathan harbours secret powers which may even rival the Master’s. When he is spotted using them he is swept up in the intrigues which blanket Mordew thick as smog.

This sprawling mythopoeia is a treat. ‘There is something about being in a place you don’t know that is both frightening and liberating,’ Nathan reflects. There is a similarly precarious licence to the storytelling. After the narrow focus of two previous novels, Lucia and Playthings, Pheby is clearly enjoying the expansiveness of genre fiction. The challenge, of course, is to carry the reader along too. An unwary writer risks vanishing down an echoing mineshaft of imagination, leaving us baffled at the top.

Pheby largely achieves that balance between invention and familiarity. The novel’s extensive glossary and dramatis personae help, as does the earthy dialogue. And the city’s filth and reek are almost Pythonesque: ‘The Master’s magic leaked down the Manse … it was putrid green and viscous, part smoke, part treacle.’ Yet as the story develops it also deepens, to consider themes of caste and inequality, agency and predetermination.

Mordew is a little loose and whirling, but it’s written with combustible verve. As with any self-respecting fantasy debut, it is billed as the first in a trilogy, and I’m curious to see where it leads.