Ben and Zoe sailed up and down between the buoys, but no alerts appeared on the BUG screen.

“I think we’re in the wrong place,” said Ben. “We’ve been searching for over an hour and there’s still no sign of Fingal.”

Suddenly, they heard a distant shout from across the water. They looked up to see a small boat out beyond the bay. They could hear the chug of its engine.

“A fishing boat,” said Zoe. “It must be coming back with its catch.”

Then there was silence as someone turned the power off. “Looks like they’ve got a problem with their net,” said Ben.

Two of the crew were desperately trying to pull up a bright green fishing net. A third person called out instructions while keeping the lurching vessel balanced.

Ben and Zoe could make out a few words, translated by the BUG.

“Something’s caught,” one voice said.

“It’s big,” said another.

“It’s struggling!” a third voice warned. “It’ll tip us over.”

Ben got out his binoculars and looked through them. “Can’t see what they’ve picked up,” he reported anxiously, “but it’s certainly in trouble.”

Zoe glanced down at Ben’s BUG screen.

“We should have checked sooner,” Zoe said. “It’s saying ‘dolphin’!”

“It could be Fingal,” said Ben. Zoe swung the boat around and edged it toward the struggling fishermen.

“The BUG identified it as a distress cry,” Ben said. “Even if it’s not Fingal, we’ve got to get in there and do something.”

Ben pressed some buttons on his BUG. “I’m saving that dolphin’s call just in case,” he said. “All dolphins have a different signature call, and if this is Fingal, we’ll be able to identify him from now on.”

Zoe steered the dinghy toward the buoys in the bay. Lowering the sails, she secured the boat to the nearest one while Ben got out their GILS and flippers.

“If only I had my sailing knife with me,” said Zoe. “We could use it to cut the net.”

“I’ve got something even better,” said Ben. “My diver’s knife.” He produced a sheathed knife from the backpack and strapped it to his belt.

“Remember what Señor Rodriguez told us,” said Zoe. “Keep an eye out for sharks.”

Ben nodded. He ripped off his life jacket and clothes. “I don’t need to be told twice,” he muttered. Ben pulled the mask on and adjusted the snorkel. Then he attached his BUG to his diving belt with a safety cord. Zoe did the same.

“Don’t forget, you only have ten minutes of air,” Zoe said.

Ben made a circle with his thumb and forefinger — the diver’s sign for OK — and plunged into the water with a splash.

The water rushed into his ears as he sank into the clear sea. Bubbles streamed up in front of his face. When the bubbles cleared, he spotted the long, dark shape of the fishing boat in the water ahead. With the GILS, he was able to breathe just as if he had an oxygen tank.

The water was very clear, so Ben made sure that he kept on the side of the boat away from the fishermen so they wouldn’t see him. But now he had a good view of the net. It was full of fish, and thrashing violently. As he came closer he could just see the terrified eye of a dolphin in the middle of the catch.

Zoe was approaching. Ben jerked a thumb up, and they swam for the surface, making sure to stay out of sight of the fishermen.

“The dolphin is badly tangled in the fishing net,” Ben told his sister as he treaded water next to her. “The more it tries to free itself, the worse it gets. There’s no way it can escape on its own.”

“Poor thing,” said Zoe. “Is it Fingal?”

“Can’t tell,” said Ben. “We’ll have to go back down to check. I’ll use my knife to cut it free, but we can’t let the fishermen see us.”

“Agreed,” said Zoe. “We’ll approach from underneath the hull — but stay far away from its propeller.”

Together, they dived under the bottom of the fishing boat.

Zoe checked for holes in the nylon mesh. After a few moments, she shook her head at Ben.

Ben held the net to steady himself as he pulled his knife from its sheath. The net gave a big lurch, and the dolphin’s head appeared as it pushed its nose desperately against its prison.

Ben tugged at Zoe’s arm. He pointed to a jagged scar running from the dolphin’s eye to just below its mouth.

They had found Fingal!