Alexei and I move slowly, practically inching our way around and around and around. But with every step, the air gets fresher, clearer. It’s boiling hot outside, but in here the air is almost chilly, and a low, rumbling hum grows louder the lower we go.

“Do you hear that?” I ask. Alexei glances back at me.

“Water?” he says, and I nod.

Eventually, we see the underground river that runs through the bottom of the massive cavern. The water is black and it feels like we’re about to reach the River Styx, but we don’t have the luxury of being afraid of what lies on the other side.

“If someone built the stairs, then they have to go somewhere. Right?” I look at Alexei, who flashes me his most boyishly charming smile.

“Of course.”

This is Alexei trying to be so positive — so overwhelmingly charismatic — that I forget to be afraid. And it might be working. Maybe. Just a little bit.

The stairs end, and Alexei puts the flashlight in one hand and takes my hand with the other then steps down onto a floor of solid stone. The river isn’t wide, but it’s deep and moving quickly. There’s a narrow path along its edge, and we walk until the river runs into what looks to be the side of a mountain. A kind of tunnel or cave surrounds it, and I can imagine this underground river cutting a path through the stone for centuries, leading away from this place — but to where, we do not know.

I look at Alexei, and, wordlessly, we follow.

“So,” he says, moving carefully along the river’s slick black banks, “how is Jamie?”

I guess we’re going to make conversation. I guess we’re going to pretend that this is just another outing — just another day.

“He’s upset,” I say. I’m not sure how else Jamie would want me to answer. Once upon a time, he and Alexei told each other everything. But now I’m not so sure.

I always thought Embassy Row’s secrets were the result of diplomacy and politics, old alliances and the new world order, but now I’m walking through a tunnel in the middle of a mountain, following a river that leads away from some sort of ancient treasure chamber. Something tells me Adria was full of secrets long before the embassies were built along its wall.

“I think the hardest part is that he wants to blame himself,” I say.

“So he blames me instead,” Alexei fills in.

I want to tell him that he’s wrong, but Alexei is a smart guy. Alexei already knows better.

“How are you?” he asks next.

I don’t say a thing.

The cavern-like path seems to go on forever. Stalactites hang from the ceiling. Or are they stalagmites? I never could keep them straight. But now I’m surrounded by them, and I know this place is not man-made. And yet, it’s easy to imagine the Romans or Byzantines or some other ancient civilization exploring this cavern — these caves — expanding, searching, looking for a place to hide.

My side hurts from so much walking and climbing, but the stitches hold and soon I’m shivering underground. We walk for what feels like hours. Eventually, the river runs away and the tunnel continues on. Sometimes it branches. Sometimes we find that it’s caved in, and then we backtrack and start again.

I have the vague sense that the ground slopes down for a long time and then back up again. Then the tunnel subtly shifts.

“I think we’re climbing,” I say. “This one’s probably caved in, too.”

Alexei nods. “Maybe it is.”

“I mean, it’s probably nothing,” I say, even though my heart is beating faster. Alexei grips my hand harder. “We’re probably wasting our time. I mean, it can’t possibly be —”

The tunnel turns — and Alexei and I freeze because, up ahead, there is a door. Not a trapdoor. Not a hidden, secret wall that spins or a tunnel that spirals. No. It’s an actual door. And, well, technically, there are two. They’re wide and made of a deep, dark wood. I reach out and touch them — they’re smooth, but, most of all, they’re real.

I want to cry, but I’m so thirsty that my eyes don’t make tears. I want to scream, but I’m half afraid someone won’t hear me.

And I’m half afraid somebody will.

“Do we dare?” I ask, but Alexei’s hand is already on the handle. And slowly — carefully — he turns.

As soon as Alexei opens the door, I know where we are. I don’t recognize the room we enter or the corridor we try to creep down, but I know. From the moment Spence and I spied the symbol on the ruins, I knew that place was linked with this one. I just didn’t realize the link would be quite so literal.

“Where are we?” Alexei asks.

“I’m not sure,” I say, and tell myself it isn’t even technically a lie.

Alexei’s hand is tight in mine. I try not to think about how accustomed I’ve become to the weight of it, the feel of it. It’s not a talisman, I tell myself. I’m certain I’d be fine without it, and yet the warm pressure soothes me in a way that I don’t dare consider long.

The corridor has low stone ceilings that arch overhead. Gaslights burn in sconces, lighting the way, but I know I didn’t see this corridor with Ms. Chancellor.

We pass old oil paintings — the kind like they have in the palace — and cabinets that are tightly locked. There are more corridors, other rooms. Alexei and I go on, following the flickering gaslight, hoping it will light the way, but the place is built like a maze, and for a second I wonder if Alexei and I have been trapped in some terrible loop — like we may never find our way out.