In Nogales we drive back into the US and leave the truck in the car park opposite Burger King, where an ancient-looking Mexican attendant accepts Duval’s $20 bill to keep an eye both on it and Taco. We cross back into Mexico on foot, along the wall, past the officer on duty, through the turnstile of the port of entry and into town. No one takes the slightest notice of us. No one asks my nationality or demands to see my passport, zippered safely into my combats. On the Sonoran side, the white concrete channel is daubed with bright religious graffiti, as if someone has bought one of the street vendors’ more lurid velour towels on their way out and iron-transferred it onto the wall on re-entry.

‘It was a school project,’ Duval says, ‘supposed to make the border look all warm and welcoming, not that anyone’s actually fooled.’

Nogales by day – a different town from Nogales by night. Gone is the unease of that night with Emmy, but the crackle of energy is still there. The main strip of souvenir shops acts like a welcome mat for tourists beyond which the goods on offer become more functional: clay pots and copper pans, multi-coloured Tupperware in every conceivable shape and size and an astonishing number of shiny bridal gowns displayed in dusty boutique windows. Duval stops to look at some pot-bellied plastic figures with shanks of orange hair in the window of a toy shop. ‘That’s the Mexican interpretation of a typical tourist.’ He points at the USA stamped on their foreheads. ‘So who says bigotry isn’t alive and well and on both sides of the border?’

It’s close to six-thirty by the time we reach the plaza and the light is softening. The fierce char-grill burn of the afternoon has dissipated into a more benign warming-oven kind of heat. We sit down on the bench nearest the church and wait.

Nogales is a people-watching town. You watch them. They watch you. A solemn group of school children pass by in grey kilts and bright red sweaters. A couple of prostitutes are loaded into the open back of a cattle truck by the police. A boy with a club foot makes his way across the square in a T-shirt so baggy it drops almost to his ankles. On a neighbouring bench a woman in a striped reboza, sitting with her child, dictates in short animated bursts to a man with pen and paper next to her.