Pulling the van into a layby just before the junction onto the M4 at Chiswick, Honor speed dials Nevis’s number, gets her voicemail, leaves a message just to say she is on her way and about to get on the motorway. She won’t be able to call for a while – nothing as sophisticated as a handsfree set – but Nevis should call or text her if she needs to and Honor will pull onto the hard shoulder and call her back.

Something has happened. Obviously. The flat has been burgled or Nevis has been mugged or, God forbid, worse. She is unwell. Or pregnant. There has been an accident. She has had a fight with Satnam. Female friendships can be scarily intense at that age. Honor knows that as well as anyone. And Nevis is sensitive and prone to misunderstandings. Growing up, all Nevis wanted was to eat chips and absorb herself in maths or birds. Most kids didn’t want to do maths or look at birds or eat chips all day and Nevis didn’t really want to do much else. The kids would go off topic and Nevis would become confused or withdrawn. Kids who were weak in maths would befriend her then get her to do their homework. At school some girl would take Nevis under her wing for a while then drop her, saying she was weird. She would go over to a new friend’s house for tea and would not be invited back. The quick allegiances and casual betrayals of teenage girls left Nevis bewildered and out of her depth. She grew more wary and lost her confidence until she arrived at Avon University to study mathematics and biosciences and met Satnam.

The van is screaming along in the slow lane of the motorway now as if it’s being tortured. A presenter on the radio, which is set to the BBC World Service, begins a lecture on Greek mythology. It has begun to rain and she is somewhere near Slough, and these facts would be depressing if Honor had the mental space to think about them. Nevis used to tell her that she drove as if she were set on murdering the engine. They would laugh about it. Like you could murder a fly, Mumtoo, Nevis would say.

Flies are safe from Honor. People are another matter. One person.