Nobody gives me a hard time for showing up late. Still, I feel guilty. With our supplies dwindling, the tension on board is as charged as a Leyden gun. For the sake of the crew, I need to be operating at 100 percent. Or at least to pretend that I am.

Our challenge arrives at 10:00 a.m., sharp.

The LOCUS display lights up with a swarm of purple splotches.

“Aircraft!” yells Jack Wu. “Wait. No…What are those?”

The purple blobs flicker in and out, changing shape and intensity. On the LOCUS, they appear to be aerial objects, hovering directly in front of the Varuna, but when I look out the forward windows, there’s nothing but open water all the way to the horizon.

Jack senses the answer before I do. He’s the best in House Dolphin at this kind of thing.

“Those aren’t physical objects,” he realizes. “See how the blobs are flattening into waves?”

I nod. “Clever.”

“What?” Dru Cardenas demands, his voice jumpy. “Are we under attack?”

He looks like he desperately wants to shoot something with his shiny new Leyden cannon. Even by Shark standards, Dru is trigger-happy. I decide I should give him a task that does not involve weapons.

“There’s no attack,” I assure him. “At least, not yet. Would you round up the other Dolphins and bring them to the bridge, please? We’ve got a code to break.”

A few minutes later, Virgil stumbles in, groggy and squinting from working the graveyard shift. Lee-Ann and Halimah follow on his heels. By the time all five of us are together, Jack and I have identified the break where the pattern starts repeating. We have also discovered how to feed the LOCUS’s electrical impulses into the bridge’s speakers, converting the glowing purple squiggles into sounds.

Halimah tilts her head. “Blue whale?”

“Partly,” I say. “But it’s more complicated than that. Keep listening.”

HP has used blue-whale song as a code for years. The pitches, sweeps, and lengths of tone can be pegged to components of human languages, making a multilayered form of encryption that is almost impossible to break if you don’t know the key.

But this code is more complicated still.

After a few seconds, the pattern changes. A series of clicks like a dolphin coda overlays the whale song. Two seconds later, the clicks and song are replaced by tones like wind through a conch horn.

Then the pattern repeats.

“The sender knows HP encryption methods,” Jack ventures. “They must assume we have a LOCUS to receive their transmission.”

“That’s good, right?” Lee-Ann says. “It must be from our base.”

“Unless it’s a trap,” Virgil says. “If this is from the Aronnax, and we reply…”

That’s a fun thought.

I find myself shaking my head. “No. This must be the base. We were expecting a challenge—”

“We were?” asks Halimah.