Before leaving Celadon, you must infiltrate the Team Rocket Hideout, located beneath the Game Corner. Talk to the Rocket Grunt guarding the poster to reveal the secret entrance. Navigate the maze-like floors, battling grunts and finding the Lift Key. You'll eventually confront Giovanni, the leader of Team Rocket. Defeat him to rescue the Silph Scope, an item crucial for later progression. This also clears the way for your next Gym challenge.
