"Whither, midst falling dew..." Illya began as he switched on the ignition.

"You said that." Napoleon checked his Wisconsin map and the direction finder. "Back down the highway," he directed. "Our quarry seems to be heading back to Milwaukee."

Illya made a U-turn in front of the house and headed for the highway. "Surely they wouldn't take the dirigible back there?"

Napoleon shrugged. "Possibly he'll turn at Madison." Sometime later he nodded in satisfaction. "He did turn. Now he seems to be angling northeast." He looked back at the detector. "And we seem to be gaining on him a little."

"Only a little?" Illya stepped harder on the accelerator. "He had to slow down going through Madison, and so will we. If he drives fast enough, he could lose us while we're getting through the city."

"Wait a minute," said Napoleon. "Now that we know the direction he's going, we can bypass Madison altogether. Turn left on Wisconsin 78—it should be a mile or two ahead—then we'll swing right and come out behind him, north of the city."

A few minutes later they were driving over a narrow, curving blacktop road. They screamed around one turn on two wheels, and Illya prudently reduced speed. "You and your shortcuts," he muttered.

Nevertheless, they eventually emerged on a major highway, and Napoleon triumphantly announced that they had gained on their quarry. "He's not more than fifteen miles ahead of us now. We should be able to cut that down a bit, on the open highway."

"And what happens if we get stopped for speeding?" Illya inquired. "Better to stay well behind than to lose him altogether."

Napoleon reluctantly agreed. "If he stays on the highway, the next town of any size is Fond du Lac. There doesn't seem to be a shortcut there; if he goes through, so do we."

Sometime later, Napoleon exclaimed, "He's stopped!"

"Where?"

"In Fond du Lac, apparently. He hasn't moved for the last ten minutes; take it easy when you get to the city limits. I wonder," he mused, "where anyone could hide a dirigible in Fond du Lac."

They swung into the city, whit Napoleon hovering over his tracer. "We're close; not more than a few blocks. We...no, he's moving again!"

"Tally-ho," Illya murmured.

They crossed the business district and Illya nodded at a gas station on a corner. "He probably stopped for gas."

"Must be. He's leaving the city now, at any rate. Drop back a bit, Illya. At this distance he could spot our headlights in open country."

Illya obeyed and drove northeast out of the city. They wound along the Lake Winnebago shore for several miles, then swung up across the ridge of land, east toward Manitowoc and Lake Michigan.

Kerry yawned. "Is he going to drive all night?

"He can't go too much further unless he takes to the water," Napoleon assured her.

At Manitowoc, the tracer pointed northeast. They followed, and had just left Two Rivers when Napoleon announced, "He's stopped again."

"This might just be it," said Illya.

"If he's stopped for gas again, he's getting very poor mileage. Drive by slowly; don't alert him by stopping."

They drove north along Lake Michigan, past a sign that announced YOU ARE NOW ENTERING POINT BEACH STATE FOREST. The highway was still winding through the forest when they spotted a rutted path leading off the highway. In the edge of their headlights, Napoleon noted a sign a few feet along the path.

"Dead end," he read, smiling. "How convenient. I do believe our Thrush is up a tree; the tracer shows him down that road somewhere. Pull on ahead and look for an inconspicuous place to pull off the road. Not too far, though, in case we have to run for it."

A few moments later, Illya pulled off the road. They got out, stretched, and began walking back toward the path. Once they flattened out in the ditch as a truck rumbled by, but traffic was light and they reached the path without further trouble.

"Do you have any idea how far it is to the lake?" Napoleon asked Kerry.

"Not for sure. I was here once a long time ago; it shouldn't be more than a mile or two from her to the water."

"All right." Napoleon gave instructions in a low tone. "We go slowly, and as quietly as possible. We'll have to stick to the road; I don't want to go blundering around in a strange forest after dark. I'll go first, Kerry in the middle, Illya last. Stay back as far as you can and still keep the person ahead in sight."

To Kerry, the advance down the pitch-black path seemed endless. They soon discovered that the moonlight didn't penetrate the branches overhead and Napoleon's plans to keep separated had to be abandoned. They kept close together the rest of the way, occasionally blundering into bushes, trees, and each other; sometimes tripping over an unexpected rock or branch lying in the trail.

At last, after what seemed several lifetimes, there was a glimmer of light ahead and they emerged from the trees onto a narrow, sandy beach. A large dark object bulked ahead of them. Careful reconnoitering proved this to be an empty boat house, with a dilapidated pier extending from behind it into the chilly waters of Lake Michigan.

The area was utterly deserted.

Napoleon consulted his tracer and waved at the lake. "According to this, our quarry is out there, not more than a couple of hundred yards from shore."

"I don't see a boat," said Illya.

"Neither do I, and I don't think he's been treading water for the last hour. Either he discovered the bug and pitched it into the lake, or..."

"Or what? If Thrush's invisible dirigible was already here and he was in it, we wouldn't be receiving any signals. Remember, any electromagnetic energy generated within the field is invisible to anyone outside the field."

Napoleon nodded thoughtfully. "He certainly didn't drive all the way up here just to throw the bug into the lake."

"He could have been picked up by someone in a boat," Illya suggested.

"In which case, we need a more versatile means of transportation to follow him," Napoleon said, pulling out his communicator. "I wonder if the Milwaukee U.N.C.L.E. branch owns an airplane, or if we'll have to get one from Chicago."

Kerry suddenly clutched his arm and pointed out toward the lake. The night sky was beginning to lighten with the approach of dawn, and the waters a few hundred yards offshore had begun to roil and bubble.

"Something's going on out there," Napoleon said, "but it's too dark to see just what."

As the sky grew lighter, the observers could make out a low, sinister shape against the water.

"Submarine!" Napoleon whispered. "There's something for Mr. Waverly!"

"You don't suppose Thrush is behind the water pollution problem?" Illya asked.

Several men busied themselves on the deck of the submarine, launching a small boat. It putt-putted in toward shore and the three watchers scrambled for cover when it became evident the boat was headed directly for the pier. By the time it arrived, they were safely concealed in a thicket not far from Sanders' car.

With the boat safely moored, the man climbed onto the pier and sauntered onto shore and up to a log that lay only a few yards from Napoleon, Illya, and Kerry. He sat down, lit a cigarette, and settled down, apparently prepared to spend the rest of the morning there. A vagrant breeze tickled Kerry's nostrils. She opened her mouth to sneeze and immediately found Napoleon's hand over her mouth and Illya's fingers pinching her nose. The sneeze subsided into a muffled gurgle which the man evidently didn't hear.