Instructions
Travel north from Vermilion City, where the boat drops you off when you first enter the Kanto region, to Saffron City.
Battle the Saffron City Gym Leader, Sabrina.
Walk east along Route 8 to reach Lavender Town, then head north from Lavender Town on Route 10 to find the Kanto Power Plant.
Enter the Power Plant and talk to the workers. You'll hear about a stolen generator part they need returned in order to get the plant working again.
Exit the Power Plant and a security guard tells you that the thief has been spotted near the gym in Cerulean City, northwest of the plant.
Travel north from the plant to find Route 9. Follow Route 9 west to Cerulean City. Visit the gym and you'll bump into the thief at the front door. When he sees you, he runs away.
Chase the thief to a bridge north of Cerulean City where he'll finally stop and enter a Pokemon battle with you. After you defeat him, he tells you he hid the stolen part inside the Cerulean City gym.
Return to the Cerulean City gym. Use the Dowsing MCHN you received in the Johto region to detect the invisible generator part on the left side of the pool, near an umbrella, chair and life preservers.
Return to the Kanto Power Plant the way you came, via Route 9, and talk to the plant manager, who stands out from the other workers in a pink shirt. He'll take the generator part from you at the end of your conversation.