The missing peice is a genorater that makes the macine inside run, When you visit the Power Plant for the first time you talk to the Operator,
(standing at the top Right of the Machine.) He will say he wanted to blast the villan who stole it with his Charge Beam. Before you exit The Power Plant, the Police officer over there will tell you about a shady character around the Cerulean Gym, After you go to the Cerulean Gym there will be a member of Team Rocket at the front of the pools he will run off, if you go to the bridge on Route 24 you will find him, He will battle you with 1 Golbat level 39, After you deafeat him he will tell you where he hid the macine part (On the left side of the pool on top of the floaty). You will get the part then go back to The Power Plant. Give the part to the Operator and he will thank you by giving you the TM Charge Beam. There is a man who wants a Dugtrio in the top left corner, he will trade you for a Magneton holding a Metal Coat it will not evolve when you do that trade though. After that you will find Misty hanging out with a nameless boy, When you showup he will run off, Misty will be angry at you but notice you wan't a Gym Battle with her so she will go to her Gym. After you swim/walk through her Gym she is waiting at the top of the stairs in the back.
I hope my walk-through helped you!