Lesbian special agent “Sully” Sullivan is a red-haired, quick-tempered Navy veteran of Irish extraction, chasing a killer through the subways of New Amsterdam one July night and having every hard-boiled detective’s proverbial bad day. The spellfire she fires through the tunnels and the patterns of cosmic geometry that help her trace her quarry immediately reveal one selling point of the world G. D. Penman builds in The Year of the Knife, where magic rather than gunpowder gives governments and revolutionaries their weapon of choice.

The fact that New Amsterdam’s late-night partygoers are in no more of a holiday mood this night of 4 July 2015 than they might be on the 3rd or 5th reveals the other: this is an alternate history as well as an urban fantasy, where Britain still rules most of North America, and New Amsterdam is Hollywood’s New York down to its grumpy taxi drivers and crowded delis, though the island of Manhattan in this metropolis has been replaced by a mysterious “Black Bay.”

Sully—a hard-drinking, womanising witch who won’t answer to her given name even when the entity calling it is a murderous immortal sealed inside the Smithsonian Museum’s strongest magical vault—is about to hunt an even more terrifying killer than those the Imperial Bureau of Investigation’s magicians usually pursue: the demon or magus that has started possessing its victims to carry out more and more horrifically spectacular murders, leaving investigators the bloody message that “this is the Year of the Knife.”

The gruesomely inventive ways that Penman finds for Sully’s adversary to send her this ominous clue—even cutting a deal with an ambitious bluegrass band to make it the title of their next smash-hit song (and, predictably, paying the devil’s price)—are one pleasure The Year of the Knife might offer fans of urban fantasy procedurals. The rule-of-cool simplicity of its magic is another: if it can look good in a fight, it can probably happen, especially if the magician casting it is Sully, who bucked against the uptight British naval college where she trained, and spends her free time (when she isn’t frequenting gin-soaked nightclubs) reconstructing a famously dangerous fire spell using “a tangled mix of Enochian, algebra, Old English, trigonometry and Norse runes” instead of the standardised Latin her instructors tried to drum in. The novel’s spellbook expands plausibly and enjoyably as the stakes rise, though it might temporarily stretch the parameters of magic too much when we first discover that the director of the IBI has been turned into a hyacinth macaw.

Sully herself is a maverick detective who battles her sexist and anti-Irish deputy director, confronts cowardly police sergeants with an “emerald stare,” flirts with receptionists, and has just started putting her days of seducing “silly young wannabe starlets” behind her by starting a long-term affair with a bisexual southern vampire called Marie. (The usual cast of urban fantasy magical creatures, we infer, exist in New Amsterdam’s world as well—and vampires are “the most loathed minority in the Empire,” to the extent that prejudice against them seems to have weakened most forms of racism and xenophobia … except what the British have projected on to Ireland.)

Occasionally, an intimate detail of their relationship helps the reader imagine what everyday life and romance in this alternate world might be, such as how normal both lovers find it when Marie is licking blood off Sully’s skin like a cat while Sully recovers from a sabre attack. At other times, the speculative sense of how women in these bodies might move through their world falls back into the background. The novel professes enough knowledge about microaggressions to have Marie react angrily to Sully (in search of a necromancer to help her interpret a clue) when she wonders whether Marie knows any from her time on the streets; Marie’s past experience (we infer) makes her hear a slur against vampire sex workers she must know all too well (“I didn’t fuck any warlocks, I didn’t finger any zombie queens for a glass of blood. I ain’t like that!”). Yet only a few lines later, she’s slipped back into sexy, playful mode. Sully herself wrestles far less than we might expect with her troubled identification with the empire and its uniformed services, having been raised in a rebellious Ireland to “hate [the British Empire] like no other”: even though this history will ultimately enable her to understand why the presence behind the killings has wreaked such devastation on the city, it motivates her earlier action and memory much less than it should.

These fluctuations are more of a problem because The Year of the Knife does seem to want to deal with the violence of empire. The demonic voices that chorus “You took everything from us … Our pain shall be your pain” and “As you fall, so shall all of your empire” when the killer strikes suggests a theme of imperial blowback, which becomes harder and harder to ignore as the book goes on. The ways the murderer harnesses magic suggest they trained in an imperial college just like Sully did, giving detective and killer mirrored resentments against the empire that trained them. Penman has described The Year of the Knife as an urban fantasy novel where the existence of magic and monsters has changed history, slowing the collapse of civilisations and empires down: if the book has set out to imagine how imperialism would exert its power in a world with magic, it’s fair to ask how well it does so.

How a fantasy alternate history speculates about imperialism and magic involves much more than a plausible backstory (in this one, a Khanate, a Caliphate, a Roman Empire, and several “African empires” persist alongside the British). The novel’s treatment of women of colour, for instance, projects racialised gazes of desire on to their bodies through the filter of Sully’s aggressive sexuality, showing a deeper layer of colonial legacies at work. Sully has an early one-night stand with a young bartender and cam-girl who has “the smooth tan skin and curviness that Sully associated with visitors from the Republic of America,” who “didn’t speak a lot of English” (we’re told this might explain how easily Sully got her to bed), and who might not even be eighteen years old—placing the protagonist’s dubious attitude towards sexual consent on the same level as her maverick workplace behaviour, which some will find a step of sympathy too far. (“Chica,” as Sully mentally nicknames her, does not even merit our narrator remembering her name.)

The introduction of a Native American skinwalker as an unexpected ally, meanwhile, references the same Navajo traditions that J. K. Rowling famously fumbled in extending the Potterverse into North America, but turns the Native envoy into “a perfect golden-skinned woman” who appears to Sully in an alleyway “completely nude—untroubled by the sea breeze, and absent of anything resembling shame.” “You people,” as Sully describes them, in fact “have an embassy,” and “don’t have to keep turning into animals and sneaking over here every time you want to share information” after all—imagining the ambassador as a hypersexualised Native woman whose people choose to use primitive magic rather than modern diplomacy because, well, they’re so spiritual. If Sully’s tastes run towards the colonial gaze—fine, perhaps, that’s how empire wants us to feel desire. But a narrative voice which could have put critical distance between the reader and the sexual politics of empire instead offers racialised women’s bodies up as spectacles, rather than showing us how empire even binds people to itself in the bedroom.

Male characters from subjugated parts of the globe fare somewhat better. Sully’s three male sidekicks are an Indian forensic examiner with a taste for questionable experiments on the dead, an English gentleman and radical academic called Leonard who’s escaped the prejudice against his “chocolate-brown skin” in mainland Britain by coming to America, and an easygoing fellow IBI agent who was born in Ophir (the African empire about which we learn most) and came to New Amsterdam as a refugee from Mongolia during Britain’s wars against the Khanate. Dr Sharma’s two extra arms, grafted on after a successful pilot experiment on doppelgangers, are the stuff of comic-book rogue scientists but could also play on Western understandings of many-limbed Hindu gods, and learning that Sharma’s experiments made “the doppelganger species … a farmed resource overnight” has nightmarish implications of industrial harvesting of another sentient species for organs on which the text doesn’t dwell. (One “maimed doppelganger” kept behind “for spare parts” provides comic relief, when Sharma stuffs him into a freezer, by “shift[ing] shape to become a penguin and bitch[ing] at him in Dutch.”)

A patient Leonard once treated in Louisiana for an inconvenient dose of immortality, meanwhile, was “a tiny Oriental man, known as the Eternal Emperor” with a doughty sumo wrestler for his translator, and a magus protecting the British governor at Roanoke is an “Indian” youth who—symptomatically—never speaks a word in an important fight scene because the revolutionary accompanying Sully fires a whirlwind of deadly needles straight into his throat. The stock similes we sometimes encounter in the narration don’t reverberate beyond their own sentences; stock characters built from colonialist tropes, on the other hand, can imply greater weaknesses in the fabric of the world.

The novel’s worldbuilding itself, in fact, fails to anticipate some of the most important things black and Jewish readers, in particular, would need to know in order to imagine projecting themselves into this world’s history. When the action reaches Carolina province, “the breadbasket of the Empire” with “wheat and cornfields as far as the eye can see,” we still cannot tell whether or not this world subjected Africans to the horrors of enslavement, but we know where a cop from New Amsterdam goes to get a good bad coffee in a place like this. The history does deal, just, with the rock on which any light-hearted magical alternate history set later than the 1930s is likely to fail—whether the Holocaust happened, and how magicians would have been implicated—by saying that a “Veil of Tears” separated Britain/Ireland from mainland Europe after the Great War, preventing there ever being a World War II. Settler colonialism does form part of these Americas’ past, although there is a separate Native polity today: one magus from the eighteenth century, a certain Arnold, is a veteran of the war against the Crow.

New Amsterdam’s eighteenth-century backstory, once revealed, in fact brings to life a myth of revolutionary liberty which is no less charged with American exceptionalism even though as far as the reader knows, it is being projected on to the continent from abroad (Penman, the back matter tells us, lives in Scotland). A cast of magi with revolutionary sympathies include a Magus Madison, a French magus called Lafayette, and (a nod to Hamilton?) a Magus Burr. Perhaps the revolution has not failed; it was just delayed. Yet we are invited to sympathise much more with American liberation (and Irish dreams of the same) than with the world’s countless other victims of British imperial power: the reader, via Sully, is asked to feel more anger for the “four hundred” citizens of New Amsterdam killed in an interdimensional disaster as the suppressed American Revolution starts to return, than for any of the unnamed victims of Britain’s own empire, whose numbers must be magnitudes more.

Even Sully’s own resolution does not necessarily fill the reader with a sense of triumph, however much they might long for a lesbian detective-magician protagonist to experience one. Her story, or perhaps just this instalment, ends with her living in a Brooklyn penthouse abandoned by a British loyalist, and Marie living with her but unable to go out in public now every other vampire in New Amsterdam has been “exiled to whichever backwater Empire would take them.” The very different levels of freedom offered to Sully and Marie could remind us, in a narrative with a different tone, that revolutions do not offer everyone the same emancipation. And yet Marie’s complaints can always still be healed by Sully’s touch. With Sully’s lover trapped in a gilded cage, and a mass deportation of a hated minority narratively brushed aside, do we really feel as if this protagonist has won? Or even believe that she’d give up being able to walk in public with her lover as long as she could occupy the penthouse she’s never appeared to covet at all? How quickly characters forget the oppression they must have felt on their own bodies is a sign of where The Year of the Knife, against its own intentions, does not speculate with its alternate history enough: projecting a convincing British Empire into a magical twenty-first century takes more than deciding how naval colleges would teach spellfire or knowing that the police still wear sabres and red coats.