In the incandescent first season of HBO’s True Detective , there is a fabulous moment where Rustin Cohle (Matthew McConaughey) takes in a wall of evidence about a serial killer who has evaded him for over a decade. Surrounding him is the flotsam of a cold case — photographs, phone records, surveillance stills, flowcharts, you name it. At the centre, like a deity at a shrine, sits a police sketch made with the help of a young eyewitness, a teenager who insists that “green stuff was hanging off his face”, a statement cracked just in time by Cohle, eventually. The “green stuff” turned out to be facial scars; just a scarred man, not a mythical monster.

The child’s expression, whether it’s visual or verbal (and here it was both), often acquires a mystical air within the confines of the detective story. In the BBC show Sherlock ’s adaptation of ‘The Hound of the Baskervilles’, for instance, the case boils down to little Henry’s childhood memories of a genetically modified hound mauling his father — memories that were, as Sherlock Holmes proves, tampered with, exaggerated. Kalpana Swaminathan’s new novel, Greenlight , the sixth featuring the detective Lalli (a female 60-something sleuth with shooting and self-defence abilities), also places a child’s painting at the heart of its mystery, the hunt for a serial child-killer in Kandewadi, one of the old ‘villages’ within Girgaum in south Mumbai. When raped, mutilated, wrapped-up corpses of children are delivered at doorsteps, the police (old Lalli faithfuls Shukla and Savio, among them) turn to Lalli for help.

Lalli’s niece Sita is back as well, as the narrator. And in this book, her romantic life becomes a part of the puzzle, with her boyfriend’s ex Seema (a caricature of irresponsible crime journalists) turning up, curious to know more about Lalli and her methods. Sita (a different word from the Ramayana character ‘Seeta’, as she testily informs Seema) remains a faithful ally — and her own deduction skills have improved considerably from the earlier books.

What makes Greenlight such a satisfactory whodunnit? To begin with, Swaminathan is a terrific writer, perhaps the best line-by-line writer of detective fiction in the country right now. The Kalpish Ratna books (which she co-writes with Ishrat Syed) are marked by precision plotting and impeccable structure. The Lalli books combine these qualities with expertly applied atmospherics — the visceral sights, sounds and encounters of a high-stakes criminal investigation. And these span the entire emotional gamut. Early in the book, Lalli relieves a grieving mother of her child’s bloodied corpse (the mother was refusing to give up the lifeless body for examination), shortly after she wordlessly snaps a bullying constable’s lathi in two. Two very different moments, and it’s a tribute to Swaminathan’s skills as a writer that they both pack an emotional wallop.

The other reason is, of course, Lalli herself. This is her sixth appearance in Swaminathan’s oeuvre and, by now, Swaminathan can’t trade off on the novelty value of a sexagenarian female detective. She doesn’t need to: Lalli is a wholly unique, thoroughly fleshed-out detective, a strange and compelling mixture of intellectual acumen, a certain cultivated, professorial severity and surprising moments of raw empathy. I read her deconstructions of criminal motives in the same rapt, breathless vein that I read the Hannibal Lecter books.

“(…) These are not the usual lust murders. True, the children were raped and tortured before being killed. But there’s a strange element here. The bodies were returned — you could say delivered back where they belonged. It tells me the crime was incomplete without this step. In the last case, Deepika’s, that’s all that happened. To the murderer, this was the most important step. Without this last act of restitution, there was no release.”

Swaminathan has always been interested in the sociological implications of crime, and Greenlight gives her the perfect platform. When a slum like Kandewadi is hit by multiple ghastly crimes in quick succession, the insecurities and bigotry of the shell-shocked residents come to the fore. Even in the face of overwhelming evidence to the contrary, they hold on to their ‘outsiders did this’ theory, refusing to believe that Kandewadi could be home to a child-killer. The fourth victim turns out to be the daughter of a sex worker, and tongues start wagging about how her child deserved it and how sex workers should not be allowed to conceive. There’s a great little passage about how Sita both wants and doesn’t want to introduce herself to the grieving mothers, “magnetised by their distress” as she is.

These are small but vital brushstrokes, moments that separate the wheat from the chaff, at least as far as detective fiction is concerned. Greenlight cements Swaminathan’s status as perhaps the pre-eminent Indian practitioner of the genre.