You have to go find the thief before you beat the gym leader and then he will tell you exactly where the missing power plant piece is, it is located in the Cerulean gym in the middle piece of water. Go to the top of the middle and press A and you will aquire the missing piece.
in the cerulean city gym
You must get the missing piece from the team rocket grunt in Cerulean city.
You have to go to the power plant which is east of cerulean( not northeast ) and go along the river. In the power plant people will say that there is a shadowy figure, or a missing piece of a machine. Go back to the gym and a person inside rushes out. He hid the missing piece in one of the floats( use surf ). Return to the power plant when you got it and give it back to the owner. Misty will then be at the gym. ;)
Go to the power plant to find that there is a piece missing from the plant. Then go to the Cerulean Gym and you'll find a team rocket grunt who will flee. Chase him north, battle him and beat him. Then you'll get a piece of info from him that'll tell you where the part is. Recover it and return it to the plant and power returns to the region.
Go in there and then go to cerulean gym rocket member shall be there and follow him and he will tell u where the missIng piece is
It is in the tubes in the end.
get the missing part from cerulean gym,and take it back to the power plant and talk to that guy who boss of the plant.
inthe cerulean gym,at some floats but first u have 2 find that team rocket dude and battle him.
Go to Cerulean City's gym and it is in the middle of the gym in the water.
go to the power plant talk to the manager then go out to the cerulean gym you will find a person there chase him he will tell you he hid the missing piece of the power plant in the Pokemon gym then you go to route 25 and find the gym leader
Find the missing part in Cerulean City's gym. It is in the middle of the gym in the water.
Beat all the gym leaders that you can fight. Then go to the kanto power plant and talk to the manager. Then find the guy who stole the piece in cerulean gym. Follow him to the bridge in route 24 and get the missing piece. Give the power plant manager the piece and go to route 25. Fight all 7 trainers and get a prize from a guy after the 7th trainer. Then go to the end of route 25 and misty will be there with a boy. Talk to the boy and he runs away. Misty gets angry at you and goes to her gym. Go to the Cerulean gym and beat her.