“Eureka Cove is a man-made lagoon partially formed from what used to be polluted wetlands,” the boat explained as we motored away from shore. “After Guidant Technologies bought this parcel of land, they engaged in a massive undertaking of environmental cleanup and reclamation. Now the Cove is a model of green engineering, and the adjoining wetlands is home to over two dozen threatened or endangered species.”

“That’s so awesome,” Savannah said.

“As elsewhere on Guidant Technologies’s Eureka Cove Campus, the cove is off-limits to gas-powered vehicles. This boat is fitted with a solar-powered battery and electric backup. It will automatically monitor battery levels and alert passengers when it is time to return to a charging station.”

“I love the future.” Eric grinned. “Okay, who’s first in the tube?”

“Gillian and me!” Savannah cried. She’d already stripped down to her bathing suit and was clipping her life jacket back on. “Come on, let’s go!”

I pulled off my shorts and left my sandals in the bottom of the boat. “Are you sure you’re okay driving alone?” I asked my brother.

“What do you mean, driving?” Eric replied. “Boat, we’d like to go tubing.”

“Please utilize location clips on passengers’ life preservers,” said the boat. “The motor will automatically cut off if passengers are thrown from the tube. Speed overrides in place. Enter the water when ready.”

The boat slowed to an idle, and we tossed the inflatable tube overboard. On top of the tube were a pair of magnetic monitors that attached by a cord to little clips. Those must be the location clips. Savannah and I hopped on the tube and attached the clips to our jackets. There was enough slack that we could still bounce around on the tube without pulling the magnets free, but if we fell off, we’d take them with us.

“This is so cool,” Eric called as the boat floated a little way away. “There’s even a monitor here for the tension on the line. I don’t need to do anything.”

“Activating voice control for engine speed,” I heard the boat say.

“Well,” he corrected, “hardly anything.”

Once the line was taut, Savannah gave the signal and the boat took off. We squealed and clung to the tube, laughing our heads off at every bounce and kick from the wake. Our path took us on a wide arc through the cove, so we could see the buildings of the Guidant campus flash by on the left-hand shore. To the right was a large, heavily wooded island, and as we got closer I could see a few buildings shrouded among the trees, as well as a rocky outcropping with what looked like a radio tower on top.

“This is awesome!” Savannah yelled in my ear. She held up her thumb to Eric. “Faster!”

The boat sped up, then began to swing us around to the right, when suddenly, the motor cut off. Momentum swished the tube closer to the boat, and we bobbed up and down on the waves created by the wake.

“What happened?” Savannah called to the boys. “We’re not out of power already, are we?”

Eric was peering at the screen. “It says something about proximity limits.”

I paddled over to the boat and hauled myself aboard. “Maybe it’s saying we’re too far off to return before the battery runs out?” I peered over his shoulder as he tapped the screen. “Oh, that’s weird.”

“What?”

“Well, all the buildings on the campus are marked on this map,” I said, pointing out the labels on the digital map on the screen. Our boat was a blinking red dot in the blue lagoon. To the left were all the streets and landmarks of the campus. “But nothing’s marked for the island.”

“Maybe nothing’s on it,” Eric said. He banged his fist against the side of the control column. “Start, you stupid thing.”

“No, there are buildings!” I said, and pointed at the island, but whatever I’d seen before now was hidden in the trees. You could still catch sight of the radio tower on the outcropping, though.

“Proximity limit reached,” the boat said. “Please specify destination.”

“We’re not going to a destination,” Eric said to the boat. “We’re going tubing.”

“Proximity limit reached,” the boat repeated. “Alternate route required.”

“Is that all?” Eric asked. “Fine then, just turn left.”

“Location clip disengaged,” the boat said. “Please reattach to continue.”

“Here,” I said, undoing my clip. “You take a turn. I don’t like watching you argue with an inanimate object.”

“Fine,” Eric said, snapping the clip to his life jacket and jumping off the side. I pressed my hand to the boat’s sensor.

“Welcome, Gillian Seagret,” said the boat. “Proximity limit reached. Alternate route required.”

“Okay,” I said. “Keep going back in the direction of the docks.”

The boat made a sharp turn to the left and motored slowly away from where Eric and Savannah bobbed on top of the tube. Once the line was taut, the boat said, “Activating voice control for engine speed.”

I checked on Eric and Savannah, who gave me a thumbs-up.

“Faster,” I told the boat, and it complied. I set us on a weaving pattern, so the tube could bop along back and forth over the wake, and smiled when Savannah and Eric screamed in delight. Howard was paying us no attention, crouched low over his notebook, his arm protecting the pages from sea spray.

“What are you doing?” I shouted to him.

“Trying to break the code, of course. But I’m not getting very far.”

“Give yourself a break,” I said. “You only got that book last night. Real code breakers use, like, computers and things to decrypt stuff.”

“I didn’t get enough of a sample to use letter frequency,” he said. “Plus, there’s no reason to think that the code is even a substitution cipher.”

I assumed that was all code-breaking talk he’d picked up from his book. “Maybe it’s not even a code,” I said. “It was just random letters and nursery rhymes.”

“Faster!” Eric called from the tube.

“Faster,” I said to the boat, which responded immediately.

“But that’s how numbers stations work,” Howard explained. “They intersperse their coded messages with songs or other messages that contain the keywords, and only the people receiving the message are supposed to know how to decode it.”

“So you think that radio station we heard was a numbers station?” I shouted over the engine.

As the boat neared shore again, it made another wide turn to head back into the lagoon, and Eric and Savannah bounced over the wake behind us.

“Are you trying to kill us, Gills?” he screamed.

“Blame this screwy boat!” I called back, then turned to Howard.

“It played numbers, didn’t it?” Howard asked. “Have you ever heard a radio station like that before?”

“If someone’s actually broadcasting code from a radio station in Eureka Cove, don’t you think the encryption would be too advanced to be broken by a twelve-year-old with his first code book?” I grabbed onto the rails as the boat veered right again. “Guidant is a tech firm. Encryption is what they do.”

Again, the boat engine cut off and I glanced up, sure I’d see that either Eric or Savannah had fallen off the tube.

“Proximity limit reached,” the boat said. We bobbed up and down on the surface of the lagoon. The prow of the boat now faced the dark island. “Please specify alternate location.”

“What is with this thing?” I asked, tapping the screen. “Why does it keep steering us to the limit?”

I checked behind us to see Eric and Savannah waving and gesturing to me. “Just a second!” I said. Behind them I could see the docks and the beach, not so far away. Not nearly as far as it had been when I had been out on the tube and we’d traveled up the shore.

“Boat,” I said carefully. “Redirect us to go parallel to the shore.”

The motor started up again, and the boat veered left. “Voice control activated.”

I checked on Eric and Savannah, then ordered the boat to speed up when they gave me the signal. As we sped away from the docks and up the coast, I ordered the boat to go faster and faster, to weave back and forth in the water so it got closer to the campus shore and then farther away, but I was always careful to keep a far distance from the island.

“Hey, Gillian,” Howard said. “Can you see if the boat’s radio gets reception for the numbers station, please? Eric wouldn’t let me.”

“You asked Eric?”

“Yeah,” he said. “We were arguing about it right before you came onboard.”

He meant right before the boat stopped. And earlier, in the car, when Howard said the words “numbers stations,” the car went and found one for him on the radio.

“Radio station,” I said out loud, to test my theory. The boat veered right, then promptly slowed.

“Proximity limit reached.”

I looked ahead of us, at the radio tower winking through the island trees. It might not be marked on the map, but there was a radio station on that island.

And, as the boat said, it was off-limits.