"Do you know if anyone has ever tried to world-walk from inside an aircraft?" Mike asked.

Matt laughed raucously.

"What's so funny?" Mike demanded.

"You Americans! You're so crazy!" Matthias rubbed his eyes. "Listen. The Clan, they know if you world-walk from high up you fall down, yes? Planes are no different. Now, a parachute-you could live, true. But where would you land? In the Gruinmarkt or Nordmarkt or the Debatable Lands, hundreds of miles away! The world is a dangerous place, when you have to walk everywhere."

"Ah." Mike nodded. "Has anyone ever world-walked from inside a moving automobile?" he asked.

"That would be suicidal."

"Even if the person were wearing chain mail? Metal armor?" Mike persisted.

"Well, maybe they'd survive…" Matt stared at him. "So what?"

"Hmm." Mike made a mental note. Okay, that was two more of the checklist items checked off. He had a long list of queries to raise with Matt, questions about field effects and conductive boundaries and just about anything else that might be useful to the geeks who were busting their brains to figure out how world-walking worked. Now to change the subject before he figures out what I'm looking for. "What happens if someone world-walks while holding a hand cart?"

"Hand carts don't work," Matt said dismissively.

"Okay. So it really is down to whatever a world-walker can carry, then? How many trips per day?"

"Well." Matt paused. "The standard corvée duty owed to the Clan by adult world-walkers requires ten trips in five days, then two days off, and is repeated for a whole month, then a month off. So that would be one hundred and twenty return trips per year, carrying perhaps fifty kilograms for a woman, eighty to a hundred for a man. More trips for professional couriers, time off for pregnant women, but it averages out."

"There's an implicit 'but' there," Mike prodded.

"Yes. Women in late pregnancy with a child that will itself be a world-walker cannot world-walk at all. Or if they try, the consequences are not pretty. But I digress. The corvée is negotiated. To a Clan member, the act of world-walking is painful. Do it once, they suffer a headache; twice in rapid succession and a hangover with vomiting is not unusual. Thrice-they won't do it three times, unless in fear of life and limb. There are drugs they can take, to reduce the blood pressure and swaddle the pain, but they are of limited effectiveness. Four trips in eight hours, with drugs, is punishing. I have seen it myself, strong couriers reduced to cripples. If used to destruction, you might force as many as ten crossings in a period of twenty-four hours; but likely you would kill the world-walker, or put them in bed for a month."

"So." Mike doodled a note on his paper pad. "It might be possible for a strong male courier, with meds, to move, say, five hundred kilograms in a day. But a more reasonable upper limit is two hundred kilograms. And the load must be divided evenly into sections that one person can carry."

Matthias nodded. "That's it."

"Hmm." An SADM demolition nuke weighs about fifty kilos, but no way has the Clan got one of them, Mike told himself, mentally crossing his fingers. They'd all been retired years ago. If the thin white duke was going to do anything with his nuclear stockpile, it would probably be a crude bomb, one that would weigh half a ton or more and require considerable assembly on site. There was no risk of a backpack nuclear raid on 1600 Pennsylvania Avenue, then. Good. Still, if James's mules are limited like that, we won't be able to do much more than send a couple of spies over, will we?

"Okay, so no pregnant couriers, eh? What do the Clan's women do when they're pregnant? I gather things are a bit basic over there; if they can't world-walk, does that mean you have doctors-" Mike's pager buzzed. "Hang on a minute." He stood up. There was an access point in the EMCON insulated room. He read the pager's display, frowning. "I've got to go. Back soon."

"About the military-" Matthias was on his feet.

"I said I'll be back," Mike snapped, hurrying toward the vestibule. "Just got to take a call." He paused in front of the camera as the inner door slid shut, so the guard could get a good look at him. "Why don't you work on the dictionary for a bit? I'll be back soon as I can."

One of the guards outside Matt's room had a Secure Field Voice Terminal. Mike took it, ducked into the Post-Debriefing Office, plugged it into one of the red-painted wall sockets, and signed on to his voice mail. The joy of working for spooks, he thought gloomily. Back at DEA Boston, he'd just have picked up the phone and asked Irene, the senior receptionist, to put him through. No pissing around with encrypted Internet telephony and firewalls and paranoid INFOSEC audits in case the freakazoid hackers had figured out a way to hack in. Sometimes he wondered what he'd done to deserve being forced to work with these guys. Obviously I must have done something really bad in an earlier life. "Mike here. What is it?"

"We got the thumbs-up." No preamble: it was Colonel Smith. "BLUESKY has emplaced the cache and on that basis our NSC cutout has approved CLEANSWEEP and you are go for action."

"Whoops." Mike swallowed, his heart giving a lurch. "What now?"

"Where are you?"

"I'm on the twenty-fourth-sorry, I'm in Facility Lambda. Just been talking to Client Zero." More time-wasting code words to remember for something that was really quite straightforward.

"Well, that's nice to hear. Listen, I want you in my office soonest. We've got a lot to discuss."

"Okay, will comply. See you soon."

Smith hung up, and Mike shut down the SFVT carefully, going through the post-call sanitary checklist for practice. (A radiation-hardened pocket PC running some exotic NSA-written software, the SFVT could make secure voice calls anywhere with a broadband Internet connection-as long as you scrubbed its little brains clean afterward to make sure it didn't remember any classified gossip, a chore that made Mike wish for the days of carrier pigeons. And as long as the software didn't crash.) "Got to go," he told the guard. "If Matt asks, I got called away by my boss and I'll be back as soon as I can."

He signed out through the retinal scanners by the door, then waited for the armed guard in front of the elevator bank. Mike gestured at one of the doors. "Get me the twenty-second." The guard nodded and pushed the call button. He'd already signed Mike in, knew his clearances, and knew what floors he was allowed to visit. A minute later the elevator car arrived and Mike went inside. It could have been the elevator in any other office block, except for the cameras in each corner, the call buttons covered by a crudely welded metal sheet, and the emergency hatch that was padlocked shut on the outside. No escape, that was the message it was meant to send. No entry. High security. No alternative points of view.

Mike found Smith in his office, a cramped cubbyhole dominated by an unfeasibly large safe. Smith looked tired and aggravated and energized all at once. "Mike! Grab a seat." He was busy with something on his Secure Data Terminal-a desktop computer by any other name-and turned the screen so that Mike couldn't see it from the visitor's chair. "Help yourself to a Diet Coke." There was a pallet-load of two-liter plastic bottles of pop just inside the door-it was Smith's major personal vice, and he swore it helped him think more clearly. "I'm just finishing… up… this!" He switched the monitor off and shoved the keyboard away from him, then grinned, frighteningly. "We've got the green light."

Mike nodded, trying to look duly appreciative. "That's a big deal." How big? Sometimes it was hard to be sure. Green light, red light-when the whole program was black, unaccountable, and off the books, who knew what anything meant? "Where do I come into it?" I'm a cop, damnit, not some kind of spook.