Zoe plunged into the pool just ahead of her twin brother, Ben. They sped through the water until they reached the hot tub, their finishing line. Zoe slapped her hand on the wall.

“I win!” she said.

“Barely,” Ben said, panting.

Zoe pushed her brown hair out of her eyes. “What should we do until the wave machine comes on?” she asked.

“More competitions,” said Ben. “Bet I can sit on the bottom of the pool and hold my breath longer than you can.”

“No thanks,” said Zoe. “You always win that one.”

But Ben had already sucked in a huge mouthful of air and dived under the surface. Zoe joined him, and they sat on the floor of the pool. Zoe kept her back to her brother, knowing he’d do anything he could to make her laugh.

All of a sudden, someone tapped Ben on the shoulder. He whipped around to see a young woman in dark goggles peering at him. She gave him a thumbs up. Ben let out all his air in a stream of bubbles and burst to the surface with a splash.

Zoe popped up a second later. “I win,” she said excitedly.

“I had to come up,” Ben said, jerking his thumb over his shoulder. “Erika’s here.”

Zoe looked up and down the pool. “I can’t see her,” she said. “You’re just making up an excuse for losing to me.”

“Over here!” came a voice with a slight German accent. Erika was standing next to a fountain in the corner and waving a glass eyeball at them.

Ben and Zoe swam across to her.

“Hey, Erika!” said Ben. “I take it we’ve got a new mission?”

Zoe and Ben Woodward were like most other teenagers, except for one important difference: they were operatives for WILD, a secret organization dedicated to saving animals all over the world. It had been set up by their uncle, Dr. Stephen Fisher.

Whenever Zoe and Ben were needed, Uncle Stephen sent his second-in-command, Erika Bohn, to bring them in. But she was never allowed to tell them the details of their mission. Instead, Uncle Stephen always sent them a glass eyeball that served as a clue to which species of animal needed their help.

“Good to see you both,” said Erika, smiling brightly. She peeled off her goggles and handed them the glass eyeball. It was about the size of a human’s, and had an inky black pupil.

Zoe turned it over in her hand. “I wonder which animal this is from,” she muttered.

“You can think about that in the helicopter,” said Erika. “Come on, let’s get dressed and head to WILD Island.”

Twenty minutes later, the three of them were flying out over the ocean. The familiar smell of chicken manure — the helicopter’s fuel — filled the air. Like everything at WILD, the chopper was environmentally friendly.

Zoe took the chance to call their grandmother while Ben examined the eyeball. Their parents were vets who were currently working abroad, so their grandma looked after them while their parents were away. Mr. and Mrs. Woodward didn’t know that their children worked for WILD. Only Grandma was in on their secret.

“That Stephen,” Grandma’s cheerful voice said through the speaker. “Where’s he sending you now?”

“We don’t know yet,” said Zoe. “He gave us a clue, though.”

“He was always one for a puzzle!” Grandma said with a laugh. “Take care of yourselves, and I’ll see you when you get back.” The speakerphone cut out.

Ben held up the eyeball. Suddenly, the pupil caught the light and glowed brightly.

“It shines,” said Ben. “You know, like a cat or a dog when you shine a light in their eyes. It helps them see in the dark.”

Zoe took a look. “What other animals have that?” she asked.

“Quite a few mammals,” said Ben. “Including one that lives underwater.”

Zoe nodded. “Ben’s right,” she said. “It could be a dolphin.”

Erika pointed toward a hole in the computer’s console. “Let’s see what your uncle has to say,” she said.

Ben placed the eyeball into the slot. At once, a hologram of their uncle appeared.

“Hello, my wonderful nephew and niece,” he said. “Have you figured out what it is yet? You might need a clue. It’s —”

The image began to flicker. Their uncle’s voice flickered in and out. “Something . . . wrong . . . hologram,” he sputtered. “Wish I hadn’t . . . spilled . . . tea . . . computer.” Then the hologram disappeared.

Zoe laughed. “Uncle Stephen may be brilliant, but he’s pretty clumsy,” she said. “I guess we’ll just have to wait.”

“WILD Island coming up,” Erika said as she brought the helicopter down for a landing.

As soon as they’d exited the chopper, Erika pressed a button on a handheld remote, activating a mechanism.

Suddenly, a fake shed rose around them to hide the helicopter. Then they made their way to an outhouse, that was, in fact, a secret elevator.

Ben and Zoe felt their stomachs lurch as they zoomed down into the headquarters of WILD. When the elevator doors opened, they found themselves face to face with their uncle. He was wearing a lab coat over bright shorts, with a funny hat atop his messy hair.

“Greetings, children!” Uncle Stephen cried. “Now come with me, there’s no time to lose.”

They hurried down the hallway. Next to a door marked CONTROL ROOM, they all took turns placing their fingertips on a small pad. As soon as their prints had been identified, the door slid open.

Now they stood in front of a large, bright room that was the headquarters of WILD. Consoles and lights flashed. Operatives clicked away at keyboards, but they all paused to smile and wave at Ben and Zoe. The two of them had developed quite a reputation among the WILD employees after all their daring rescues.

Uncle Stephen led Ben and Zoe over to his desk. “Sorry about the hologram,” he said. “Were you able to figure out what animal the eye belongs to?”

“Well, we’re pretty sure it’s a mammal,” Ben said.

“And?” Uncle Stephen asked.

“That’s not much to go on,” Erika said. “Maybe you should give them another clue.”

Uncle Stephen stroked his chin. “Although this animal is a mammal,” he said, “it lives in the sea, and it’s particularly intelligent.”

Ben opened his mouth to speak, but before he could, Zoe cried out. “I knew it,” she said. “We’re rescuing a dolphin!”

“Nice work,” said Uncle Stephen. “I knew you two would discover the answer!”