It is always a strange dynamic, being a nanny for a family. The battle of power, parents relinquishing control for the first time… it can take a little while to adjust.

Leïla Slimani’s novel Lullaby exploits this tension and makes it take a turn for the sinister.

I’m not giving anything away when I say that this is a very dramatic and horrific story (the strapline on the cover says a lot!):  a nanny called Louise comes to work for a family and kills the children. This much is explained to us on the few pages, so it raises questions: what has happened to drive her to murder? If indeed, anything at all?  Why did she do it?

The story goes back in time to show us Louise’s arrival. Myriam and Paul are a hard-working married couple living in Paris: Myriam is a high-powered lawyer, her husband works in the music industry. They have two young children and Myriam stays at home to raise them at first, but feels guilty for missing her career. She wants her job back.

Enter Louise, a seemingly perfect and committed nanny for whom no job is too big or small. She tidies their apartment, she cooks the most delicious meals, the children adore her. It’s not all smooth-sailing, however: we get a very clever handling of those normal/awkward moments one would expect with a new adult in a family house. But we are aware that these are the shadows of tragedy: we know that much, much worse is yet to come.

The story has very little dialogue, particularly from Louise, so her motives are suspended tantalisingly beyond our reach. But we do know quite a lot about her: she is a mother to a grown-up daughter who she has lost touch with; she was in an abusive relationship. We learn that she has ‘never listened to music.’ She is delusional. She has mad outbursts, a bad temper. She behaves strangely: she fishes a chicken carcass out of the rubbish bin for the children to play with. But this is uncomfortably off-set with perfect behaviour: she arrives early and stays late, for no extra money. Myriam and Paul cannot imagine life without her. Until, suddenly, they realise that they can, and should, try and manage without her.

We get flashes forward in time to people who knew the family, neighbours and Louise’s old employers, telling their side of the story to the police. It is all told in the present tense, giving the narrative a breathless, pacy feel.

Louise is a creepily fascinating character: she is described as psychotic, child-like in her permanent Peter-Pan collar. She is obsessional, frantic, deranged.

Lullaby is a great read, griping and tense: thankfully it’s not too gory, like Hitchcock, Slimani leaves much of the horror to our imaginations.

Lullaby by Leïla Slimani was published by Faber and Faber in January, £12.99 paperback.