Imogen West-Knights [Alex Krook]

“PARIS has beneath it another Paris,” Victor Hugo wrote in Les Miserables.

Billie and her brother Tom have both, in their own ways, sought to find themselves and come to terms with family trauma in the French capital.

But until they reach the catacombs, it’s all been a bit bathetic. After losing their friends in the tunnels, all they have is each other – something they’ve been avoiding, even when in close proximity, since coming of age.

Deep Down begins, in narrative terms, on an aeroplane, with Billie dropping her suitcase on an old woman’s head. Following the death of her father, flying out to Paris seemed the natural thing to do, and nobody should count on travelling by budget airline without accruing additional trauma en route.

We are soon transported back to Billie’s previous life in London, and to “International Parkour Week” in her marketing office, where everyone tries to look busy.

The other account occupying the assembled great minds is “Vapinity”, a new brand of vape.

It’s the kind of detail on which Imogen West-Knights thrives. Characters are introduced with damning but throw-away assessments, such as Laura, “who once threw farewell drinks before going on a three-week holiday”, and Elle, “an accomplished university gymnast who is very good at finding ways to wedge the fact she is tiny into conversation”.

The press release accompanying Deep Down touts it as “perfect for fans of Naoise Dolan, Katherine Heiny and Megan Nolan”.

The 55-year-old Heiny notwithstanding, it’s a clear if lazy pitch to the booming market for millennial fiction. This is West-Knights’s first novel, but the author has already achieved a cult following as Britain’s foremost millennial feature writer. Her articles on everything from the assassination of Olof Palme to a Gone Girl-themed cruise ship have a tendency to delight and disturb in equal measure, and the same is true of Deep Down.

For interspersed among the ironised observational comedies are far darker episodes, dealing with Billie and Tom’s childhood, and the mysterious illness and violence of their father. Their mother, aunt and stepmother all have issues of their own, but as is so often the case in real life, we are frequently left guessing.

Billie herself is haunted by her father’s ghost and that of her namesake, the actress Billie Whitelaw, who in later life could occasionally be found hoovering the stairs of a terraced house in NW5’s Spencer Rise.

West-Knights casts an adult’s ear to the hopes and fears particular to the young child’s mind, and the result is a thing of beauty.

“Tom follows the path of one raindrop, slithering down the window beside him, gathering speed and size as it races towards the bottom to vanish into the rest of the water settled there,” she writes.

This tear-like trajectory reflects how the siblings have both repressed and magnified their troubles, seeking reassurance in the world moving on around them, but remaining stuck themselves.

After being guided into the catacombs by Noémie, a masterfully crafted fearless Parisian with “anime-red” hair, the siblings find themselves separated from the group, and their ever-present tensions bubble over into rage. What could be a moment of clichéd reconciliation is pulled back to a more realistic mutual recognition, as they deal with abandonment in a cold, dangerous place.

Billie later “knows that neither of them have the energy to explain exactly what they’re sorry for”, but they have, at least, finally felt each other’s hurt as well as their own.

Deep Down is a novel about discovery, after all, but not in the way you’d expect. The greatest truth, it seems to say, is that of inadequacy.

Healing can come through accepting each other’s flaws, and through coming to terms with the fact that repression is sometimes a necessity.

A human’s subterranean mirror image, like that of a city, promises transformation – but more often than not, we realise that our greatest discoveries were always there above ground, before our very eyes.

• Deep Down, by Imogen West-Knights. Fleet, £14.99