The first few plaques were hard to find in the Barbican’s three-dimensional concrete maze of roads and highwalks — “Like an inside-out prison,” as Linda put it. The first plaque was glued to the wall of a modern bank building; it showed the site of a late Roman city gate, now long demolished. By the time we got there Linda was already sweating. “Is this going to be the story of the afternoon? Crappy little plaques showing where things used to be?”

“What did you expect, Gladiator ? …”

The next few plaques led us around the perimeter of the old Roman fort. Stretches of the wall were visible in scraps of garden below the level of the roadway. Much of the wall had been built over in medieval times, then uncovered by the archaeologists. The ground level had risen steadily over time; we walked on a great layer of debris centuries thick, a measure of the depth of time itself.

Plaques seventeen to fifteen caused us some arguments, because they were scattered around the ruins of a round medieval tower set in a garden in the shadow of the museum itself. We traipsed over the grass- covered ground, to the water and back, trying to figure out the peculiar little maps that supposedly showed us how to get from one plaque to the next in line.

Plaque fourteen was in a churchyard that turned out to be a little oasis of peace, set away from the steady roar of the traffic. We sat on a bench facing a rectangular pond, bordered by concrete. The wall, with its complex layers of medieval building and rebuilding, stretched its way along the bank opposite us, passing the remains of a round fort tower. I’d brought a couple of bottles of Evian, one of which I now passed to Linda. She had been right about the shoes. My arches were already aching.

“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.

She asked me, “So what will you do? Will you go to Rome?”

Lou had suggested that, but I still wasn’t sure. “I don’t know. It seems like a big commitment—”

“ — to a project that might be completely wacko. But it might be the only way you’re going to be able to clear this up, if you’re serious about it.”

“I’m serious. I think. I don’t know.”

“Same old same old. George, you’re a good man. But you’re so fucking indecisive. You blow with every breeze.”

“Then you were right to kick me out,” I said.

We walked in silence for a while.

Plaque four was at the back of an office building — we had to be bold enough to walk into private grounds — where we found a sloping glass frame, like a low greenhouse, set over a trench in the tarmac. A section of the wall was exposed, twenty feet deep under the glass, through which we peered. We couldn’t see the lower section, the Roman bit, because the office workers in their dungeon below had stacked boxes and files against it.

I was the first to say it. “Okay, I’m sorry. But I’m really not sure I need advice right now. My family may have been something of a screwup, but there’s nothing I can do to change the past. And now it’s gone — Gina’s fled about as far as she can go — and all I’m left with is …”

“This loose end. Which you can’t resist tugging. Well, I think you should go. Let’s face it, the death of a parent is as big a loss as either of us is ever likely to face. I think you should take some time to get through it. And if this sister thing is an excuse to do that, fine. Go to Rome. Spend some lira.”

“Euros.”

“Whatever.”

“I was sure you’d try to stop me.”

She sighed. “Listening is just one of the skills you never acquired, George.” She touched my hand; her skin was warm and comfortable. “Go. If you need anything just call.”

“Thanks.”

“Now let’s finish this stupid walk.” She marched on.

We passed out of the City area and came down Cooper’s Row, passing under the rail line into the tourist- oriented area close to the river, and the Tower. We passed through the Tower Hill underpass by the Tube station entrance, peered at the ruin of a robust-looking medieval gate, and then walked back through the subway to where a sunken garden at the northeast corner of the underpass contained the statue of an emperor — and, ironically, right at the end of the walk, the best-preserved section of wall we’d seen all day.

We sat on a bench and sipped our water.

“One more tick for your little book,” Linda said, not too unkindly.

“Yep.” It was all of thirty feet high, and the Roman section itself maybe ten feet. The Roman brickwork was neat rows interspersed with red tiles that might have come from my father’s house. The medieval structure above it was much rougher. “If I didn’t know better I’d have said that the Roman stuff was Victorian, or later,” I said. “It’s as if the whole wall has been turned upside down.”

Linda asked, “Civilization really did fall here, didn’t it?”

“It really did.”

“I wonder if she came here. That great-grandmother of yours. Regina.”

“… And I wonder if she knew that it would all disappear, as if a small nuclear bomb had been dropped on the city.”