It was at Beverly Hills that I met Mash (short for Macharia), who reintroduced me to Nairobi. We would walk together down Moi Avenue, the street that leads from Nairobi the international city to the undocumented sprawl of an evolving African city: people and their small, illegal constructions fronting opaque skyscrapers; secondhand-clothes shacks and rickety vegetable stands; wooden cabinets behind which whispered price-setting over watch repairs takes place in Dholuo, the language of Lake Victoria; shoe shiners and shoe repairers soliciting work by keeping eyes on the feet of passersby. These people tell tall political tales that later turn out to be true.

Mash was in his late 20s. His father had been a wealthy man. He was, to Mash, a man living in English, who believed in education and "fair play." A man who invested a lot of time telling his children to look forward, to the West, to progress. Then he died, and at his funeral another wife and three children appeared, as if from nowhere. Mash's father had managed to hide a family for 20 years.

In order to negotiate our complex lives, Nairobi people have learned to have dual personalities. We move from one language to another, from one identity to another, navigating different worlds, some of which never meet.

Mash would go to work in the morning for a tour company, where he spoke good private-school English. In the evening we would cross to Biashara Street in Mlango Kubwa to drink and talk. We would speak in English about philosophy or literature or the formal job market. We would speak in Kiswahili about life in general, about the little things that made up our day. We sought a kind of brotherhood from our conversations in Kiswahili—speaking always in a mock-ironic tone, laughing a lot, being generous about each other's opinions, offering each other drinks and favors in ways we could not in English.

We hide whole lives in the gaps between these forked tongues. This is how Mash's father managed to hide his village family for so long. He was somebody else, somewhere else, in another language. His story is not unusual.

Mash seemed to know everybody and have a thousand deals running at the same time. He had shares in a small shop selling mobile phone airtime; some weekends he would go up-country to buy mung beans to sell in Nairobi. Much of the money he made was spent on lawyers. He felt it was his responsibility to restore to his mother items he believed various relatives had stolen from her.

I saw him in action one day in Mlango Kubwa when we stumbled upon a group of women, secondhand-clothes dealers, who had caught a thief—a dirty, disheveled young man, eyes bleary from sniffing glue. A crowd had already gathered. They were ready to beat him to death.