After finding a hidden switch in Rocket Game Corner, you discover the secret Team Rocket Hideout down below.
Floor B1F
Go south from the entrance and talk to the Grunt to get a Team Rocket Set for your Clothing Trunk.
Go northwest and you will battle a Team Rocket Grunt who has a level 29 Golbat. You get 812 Pokédollars for winning.
Go southwest from there to find a set of 5 Ultra Balls.
From there, go north, then east as far as you can go (but not down the stairs), then south, where you can battle a Team Rocket Grunt who has a level 29 Koffing, a two level 29 Rattatas. You get 812 Pokédollars for winning.
Go south from there to find a set of 3 Mighty Candies.
Then go to the north part of the room and go down the stairs on the right.
Floor B2F
Go west and you will battle a Team Rocket Grunt who has a level 29 Hypno. You get 812 Pokédollars for winning.
Go west past the stairs. Step on the lower of the two green conveyor belts. When you stop, go to the left, and make your way east (avoiding the conveyor belts) to find a Nugget.
Then go west and walk over the yellow tile that you stopped on earlier. Go south from there to find an Awakening.
Next, step on the uppermost of the green tiles that is pointing to the right (the one that is to the right of the Awakening that you just picked up). When you stop, go south to get the TM05 Rest that is nearby.
Then walk east over the yellow tile that you just stopped on, then go south, and you will have three green tiles around you. Step on the left one. When you stop on a yellow tile, go to the right to step on the green tile that is directly to the right of the yellow tile. When you stop, go to the left to step on the green tile that is directly to the left of the yellow tile. Then go east, then south, and step on the lower of the two green tiles. When you stop, south and follow the path to get a PP Up.
From there, go east, then walk north over the yellow tile, and check the ground to find a hidden X Accuracy.
Then go to the left and step on the green tile, then go north and step on the tile that is pointing upward. When you stop, go east, then south, and there will be three green tiles around you again. This time, step on the one on the right that is pointing downward. When you stop, go to the left to step on the tile that is pointing leftward. When you stop, go south and step on the uppermost of the two green tiles. When you stop, step on the tile to the south.
Then go east, and you will find an elevator, and some stairs leading up. You can't use the elevator right now, so go up the stairs.
Floor B1F, Second Area
Go north, and go to the right to find a set of 5 Great Balls.
Go west and you can battle a Team Rocket Grunt who has a level 29 Ekans and a level 29 Arbok. You get 812 Pokédollars for winning.
To the left, there is a hidden Full Heal in the cardboard box to the left of the office chair.
Go west from there and you can battle a Team Rocket Grunt who has a level 29 Voltorb. You get 812 Pokédollars for winning.
In the northwest corner of this area, there is a Hyper Potion.
Go east and go back down the stairs.
Floor B2F, Part 2
Go west and step on the green tile to the north. Go east and down the stairs to the left of the Team Rocket Grunt.
Floor B3F
Go south and you will battle a Team Rocket Grunt who has a level 29 Golbat. You get 812 Pokédollars for winning.
Go south to find TM20 Dark Pulse.
Check the cabinet in the southeast of this area to find a hidden Guard Spec.
Then go north, then west. Keep going west to find a Revive.
Then go east, then south when the path splits. Go west from there, then step on the tile on the left. When you stop, go south and step on the tile there. When you stop, go toward the west to find an X Speed.
Go south and there will be two green tiles that are pointing to the right. Step on the upper tile.
When you stop, go south and follow the path to the right, where you will battle a Team Rocket Grunt who has a level 29 Rattata and a level 29 Voltorb. You get 812 Pokédollars for winning.
Go east and down the stairs.
Floor B4F
Go west to find a PP Up and a Hyper Potion.
Then go north and talk to the Team Rocket Grunt, who has a level 29 Grimer. You get 812 Pokédollars for winning.
After the battle, the Grunt throws the key onto a walkway. Talk to the Grunt and you will look at a grate in the wall to the left of the fire extinguisher. Check on that grate, then you will need to find something to climb on. Then check on the office chair to put it in front of the grate. Finally, check on the chair and you will switch to your partner pokémon, who is up in the vent. Go north, then west, then south and check on the grate to push it open. Then go west on the purple pipes, then south, then east on the raised walkway. When you stop at the purple pipe, check on it to go across. Then go east to get the Lift Key.
There will be a cutscene. Afterward, go to the left and have your partner pokémon check on you. You will be reunited, and you will have the Lift Key.
Go south, and up the stairs.
Floor B3F, Part 2
Go to the left, then go north and step on the green tile. Then get the Rare Candy to the right. Go west across the yellow tile, then go north and step on the tile that is pointing up.
Go east, then go up the stairs.
Floor B2F, Part 3
Go west and step on the lower of the two green tiles that are pointing to the left. When you stop, step on the nearby tile that is pointing to the right. When you stop, go east, then south, where there are three green tiles next to you. Step on the one on the right. When you stop, go left and step on the green tile that is pointing to the left. When you stop, go east, then south, and step on the upper of the two tiles that is pointing to the left. Then step on the tile south of you.
Go east and go into the elevator, first go to B1F and battle the nearby Team Rocket Grunt, who has a level 29 Zubat and a level 29 Gastly. You get 812 Pokédollars for winning.
This will open the locked door to the left, allowing you to go north and return to Celadon City to heal up before you challenge the Team Rocket Boss.
When you are ready to continue, go back into the elevator and go to B4F.
Floor B4F, Part 2
Go west to find an Elixir.
Then go east, then north, and you will have to battle Team Rocket Jessie and James. You will have a double battle. They send out a level 32 Weezing and a level 32 Arbok. You get 3072 Pokédollars for winning.
You can heal up again if you want to. When you're ready, go back to B4F and go north from the elevator.
In the Boss's office, there will be a cutscene. Then you have to battle Team Rocket Admin Archer, who has a level 33 Weezing and a level 33 Golbat. You get 3300 Pokédollars for winning.
Once again, you can leave and heal up if you wish. When you are ready, go up and talk to Giovanni.
Team Rocket Boss Giovanni has a level 35 Persian and a level 35 Rhyhorn. You get 5600 Pokédollars for winning.
After the battle, you rescue the Cubone and receive the Silph Scope.
Before you leave, check in front of the flowers in the upper-right corner to find a hidden Max Revive.
After the Team Rocket Hideout
When you leave the Game Corner, there will be a man outside with a balloon contraption. Talk to him, say Yes, and your partner will learn the Secret Technique Sky Dash. This is like Fly in previous Pokémon games. You can now instantly go back to any city or town that you have visited before. Just go to the menu, go to "Play With Eevee" or "Play With Pikachu", choose Secret Techniques, and choose Sky Dash. Then you can select which location to go to.
You can use Sky Dash to return to Pewter City, where you can use Chop Down to reach the office on the side of the Museum. In there, you can talk to a scientist to get an Old Amber. You can't do anything with it yet, but you will be able to later.
Use Sky Dash to go back to Lavender Town. Go toward Pokémon Tower and there will be a cutscene. After that, go up to your rival and agree to go together. When you're ready, enter Pokémon Tower.