“You know, I used to have a toy like this,” I said. “A castle, I mean. It was all plastic, a base with cylindrical towers and bits of walls you snapped into place, and a drawbridge for little knights to ride in and out . . .”

She leafed through the walk guidebook. “I can’t believe you’re actually ticking off the plaques as we find them. You’re so anal.”

“Oh, lay off, Linda,” I snapped back. “If you want to pack it in—”

“No, no. I know how you’ll fret if we do.” Which was code for her saying she was vaguely enjoying the little expedition. “Oh, come on.”

We walked on.

As we counted down the plaques, we passed the sites of vanished city gates and found more sunken gardens set away from the road, like islands of the past. But as we headed down Moorgate the plaques were less interesting, spaced farther apart and set on office walls. Moorgate itself was a bustling mixture of shops and offices, with, as ever, immense redevelopment projects going on. We had to squeeze our way on temporary walkways around blue-painted screens, scarily close to the unrelenting traffic, while intimidating cranes towered overhead.

One of the prettier sites was another little garden area close to the entrance of All Hallows Church: office workers sat around, jackets off, smoking, their cell phones glistening on the grass beside them like tame insects. But the plaque—number ten—was missing from its plinth, probably long ago vandalized and never replaced. Number nine was gone, too, and number eight seemed to have been swallowed up by redevelopment. My little book was acquiring frustratingly few ticks. The walk itself dated back to 1985, long enough for time and entropy to have started their patient work, even on the plaques.

I asked, “So why did you want to see me?”

Her eyes hidden by her Ray-Bans, she shrugged. “I just thought I should. Jack’s death . . . I wanted to see if you’re coping.”

“That’s good of you.” I meant it. “And what do you conclude?”

“I guess you’re healthy. You still have that damn duffel coat, and your sphincter is as tight as ever . . .” She turned to me. I could see her eyes, flickering in the shadows of her glasses. “I’m worried about this quest to find your mythical sister.”

“Who told you about that?”

“Does it matter?”

“I suppose you think it’s anal again . . .”

As we approached Aldgate, we were entering the financial district of the City, the area where I had spent so much of my working life. At this time of day, late afternoon, the pavement was crowded with people, mostly young and bright, many with cell phones clamped to their ears or masking their faces. It felt genuinely odd to be tracing the wall, this layered relic of the past, through a place that was so bound up with my own prehistory.