Roxy followed the humming sound to Dr. Dean McGreen's farm. A dog sat in front of the barn.
"You haven't been humming, have you?" Roxy said.
"Woof!" Walden gave a friendly bark.
"Why are you barking, Walden?" a man said. "Oh. Rocket Girl."
Roxy recognized the man. "You're Dr. Dean McGreen—the scientist studying clean energy."
"That's me!" Dr. McGreen said. "And my work is paying off. I've invented the perfect way to protect the environment. Let me show you."
Dr. McGreen took Roxy inside the barn. A big machine was humming. It had a large chamber.
"I was developing a new clean fuel," Dr. McGreen said. "But I had a little accident and some chemicals spilled. When I woke up, I had exciting new powers!"
"What do you mean?" Roxy asked.
Dr. McGreen stared at a lawn mower. His eyes glowed. Suddenly the lawn mower turned on and zoomed around.
Walden whimpered.
Dr. McGreen's eyes glowed again, and the lawn mower stopped. "I can control any gas-burning engine with my mind now." Dr. McGreen laughed.
"You stopped all the cars!" Roxy said.
Dr. McGreen nodded. "The air in Metro City is cleaner than ever. And now I'll do the same for the whole world."
"Huh?" Roxy said.
"Right now I can stop cars only in Metro City. But this machine will spread my power farther. When I step inside, I can turn off all gas-burning engines—everywhere!"
"You can't do that," Roxy said. "We still need some of those cars. Maybe you can keep working on your ideas. You'll find a better way to stop people from using gas soon."
Dr. McGreen frowned. "I thought you would understand because you're also a hero. But it's fine. I'll do it on my own. And you won't stop me."
Dr. McGreen's eyes glowed again. Vroom! Vroom! A tractor sprang to life. It came straight for Roxy.
She darted away. The tractor quickly turned around and came back toward her!
"Grr!" Roxy punched the tractor. She smashed it to pieces.
Dr. McGreen climbed into his machine. His whole body glowed.
A car suddenly started. As it shot forward, Roxy grabbed its front bumper. The car's rear tires screeched.
Roxy flipped the car over. The wheels spun in the air, but the car couldn't move anymore. "Yes!"
Vroom, vroom! The lawn mower sped toward Roxy. She leaped out of the way.
Suddenly Walden barked. Roxy looked back. The lawn mower was zooming toward Walden!