Instructions
Go to the side of Route 10 that has a Pokemon Center. Heal your party of Pokemon at the Pokemon Center and head up Route 10 until you find a body of water. Surf north and go inside the Power Plant on Route 9.
Talk to the manager, and he will tell you about the missing generator part. Head toward the exit of the Power Plant, and the guard will tell you that civilians last spotted the thief in Cerulean City.
Go to Cerulean City and walk along the bridge. Talk to the Grunt that is trying to hide himself in a large group of people. Defeat the Grunt, and he will tell you where he hid the machine part. Head to the Cerulean City gym.
Enter and walk to the upper-left side of the gym. Stand near the giant lifeguard tower and interact with the buoy by pressing the A button. Your character will discover the machine part, and he will place it in his inventory. With the machine part in the bag, head back to the Power Plant.
Walk inside the Power Plant and look for a chubby man, who you will find on the ledge. Talk to him, and your character will hand over the machine part. Wait for the man to reinstall the part to get the power going again. The man will give you TM 57 for helping out.