"The yellow-backed thrush, I think it is," Solo answered.

When their turn came, the guards seemed to inspect them with extra care. Solo felt sweat on his eyelid under his sunglasses. Finally one guard slapped a ticket into Solo's hand. He jerked his thumb at the turnstile. Solo and then Illya passed through.

As they wandered over to the parapet on the sea side of the castle courtyard, Illya said, "I saw the second guard in the booth turn some sort of switch. Probably a scanner. Lucky we didn't fetch our guns along."

Solo shrugged. "I suppose. But I don't feel very secure just armed with potato chips."

Along the parapet tourists leaned over for a dizzying view of the cliffs on which Castle Sykedon was built. Napoleon Solo and Illya Kuryakin joined in the activity. Their interest was more specific. They kept up a running line of inane chatter while their eyes ranged back and forth, hunting for some trace of shadow down at the base of the cliff hundreds of yards down. A brisk sea was running. Waves crashed in and foamed onto big rocks far below.

"Nothing shows," Illya whispered after a moment.

"Good old Supervac Twenty-two-Q does it again," Solo said disgustedly.

Suddenly, though, his attention was caught by a dull metal flash under the surface of the water directly below him. The patch of water in question roiled between two partly submerged boulders which were farther apart than most of the other rocks. By a freak of the tides, the passage was momentarily untroubled by waves. Solo had a chance to take a lightning quick second look.

Illya watched him intently. Solo whipped his camera from around his neck, pretended to snap a soaring gull. Behind the camera's cover, he said, "I saw it. Some kind of protective wire grill or grating in that channel. Under the surface. Probably electrified, raised and lowered from inside."

With a fatuous look on his face, he finished fooling with his camera and turned back to the courtyard. Sure enough, one of the uniformed guards in the booth was watching them. Solo gestured at the high stone steps leading up into the castle. Pretending to laugh, he said, "I think we've found our bird's nest. That's a net guarding a sub pen entrance down there. Let's go to work."

Their feet clacked as they passed into the gloom of Castle Sykedon. A placard near the entrance announced that the castle closed for the day at six. Less than three hours.

At various points in the corridors and huge, vaulted halls, stiff-backed guards in neat, undistinguished uniforms stood with hands laced at the small of the backs, staring ahead at nothing. Their eyes never seemed to move, but Solo had the uncanny feeling that no visitor went unobserved. From the tough jaw-lines and broken noses of some of these specimens, Solo felt even more sure that they were in a THRUSH installation.

In the huge main dining hall of the castle, Illya and Solo drew off into a corner and pretended to admire an intricately wrought suit of armor. The light was dim, falling through high open slit windows. Only one THRUSH guard watched this chamber. He was stationed at the entrance.

Solo took a small sketch pad and charcoal pencil from his gadget bag. He penciled the words Need hide till six on the pad. Then, with bold, quick strokes, he made a sketch of the casque of the armored figure on top of the message, obscuring it. As he and Illya left the chamber Solo noted that the guard's eyes slid around to get a clear look at the sheet of paper on which he was still doing some shading.

They explored further, moving in and out of the crowds of families, widows, of schoolchildren. In one cobweb-hung cul-de-sac on the main floor, the agents spotted a large, old wooden chest. It looked large enough to accommodate both of them. Illya sat down on it. He complained loudly about smarting bunions.

By shifting his weight back and forth Illya was able to determine that the lid of the chest was not nailed shut. Solo sat down next to him, made some more corrections on the armor sketch. He used his scribblings to jot some additional code words suggesting his plan of action. Illya nodded. A guard passed by the entrance to the cul-de-sac, pausing to tie his shoe.

"Well, Wilbur," Solo said in a loud voice, "let's get this show on the road." Settling his fez at jaunty angle, he marched over to the guard. "Is there a pop concession any place around?"

"No, there isn't." The guard had a rumbling voice and a right ear which bore an ugly scar.

"Well, guess I'll just have to eat some of my own potato chips." Solo pulled out a cellophane sack. Making a great show, he tore it open. The guard scowled as Solo and Illya began to munch chips by the handful, dribbling a trail of crumbs behind them.

Up ahead, a number of stout women in floral print dresses and picture hats had stopped before an impressively carved throne chair. One of their number was reading from a guidebook. Solo dug in the bottom of the potato chip bag until he found the pellet he wanted.

He drifted around to a position on the left side of the many-chinned lady reading from the guidebook. Suddenly from the back of the crowd, Illya said, "Oh, I'm very sorry. Terribly clumsy of me—"

Several of the ladies moved out of the way. Illya had spilled his potato chips in grand and crumby style. Heads turned front. The portly lady stood on tiptoe and abandoned the guidebook a moment. Solo cracked the pellet with his thumbnail, and tossed it.

The pellet rolled along the floor, where it lay unobtrusively a few inches behind the lady's right heel. Nothing visible could be seen escaping from the crack in the pellet. But in another moment, the lady began to fan herself with a lace hanky.

"Girls! Girls!" she called. "Time to move on to the next point of interest, which is King Woglyn's water closet—ahem. I believe that's one point of interest we might pass over. I—"

The woman's eyes grew glassy. She dropped the guidebook, swaying. "I feel—faint. It must be the sea air doing it. My legs—I can hardly—" Three of her compatriots rushed forward, luckily catching her bulk before it hit the floor. Consternation gripped the ladies.

Clutching his fez, Napoleon Solo raced back to the guard stationed at the entrance to the cul-de-sac. "A woman's fainted over there."

The guard seemed reluctant to leave his post. The flustered outcries of the females changed his mind. Annoyed, he stalked forward and into the group.

Other guards appeared. Illya Kuryakin had backed away. Solo gave a quick nod and both agents faded quietly down the cul-de-sac. Moments later, they lowered the lid of the huge chest.

Solo tore the cover of a paper match packet in half, carefully wedged it under the lid. The tiny crack thus produced was enough to enable him to see what was happening outside.

The lady who had fainted from the fumes of the invisible and relatively harmless nerve gas was carried out of sight. Her cohorts followed. Soon the guard was back on duty at the cul-de-sac's entrance. He glanced around suspiciously. Hardly breathing, Solo and Illya crouched in painfully cramped positions inside the chest.

Ten minutes passed.

Fifteen.

Surreptitiously, the guard got out a cigarette and lit it. Cupping it so that it was hidden in one hand, he walked over to the chest, sat down heavily. Solo's view was cut off as the guard's weight bore down.

After a seemingly endless time the guard got up again. Voices drifted faintly through the chest walls. Finally, when it seemed as though he couldn't stand the ache of his pretzel-like position any longer, Solo noted that the illuminated hands on his watch stood at a few minutes past six.