Take the path north. You'll be stopped by Professor Juniper and Bianca. After the three of you talk for a bit, you'll be dragged over to a building with an old woman standing in front of it. She'll head inside and the three of you will follow. You'll talk some more then they'll rush back outside
Head outside, talk with them, answer a question (it doesn't matter what answer you give) and they'll eventually leave, heading south under a bridge. From here, continue down and enter the Pokemon Center. There's an interesting woman to talk to here.
If you show her one of the badges you've earnt, she'll tell you what Pokemon your team consisted of when you won that badge. You can do this as much as you want. Heal your Pokemon and stock up on any items you might need, then leave the Center.
Head up onto the bridge to the right of the Center. Follow it around, past the vending Machine and over the road. There's a Full Heal hiding to the left of the building located at the corner before the path turns north. Grab it then continue on north.
Follow the bridge around until you reach the Black Belt near the house at the end. Enter the house and grab the Metronome from inside. Head back to the Pokemon Center, then head down the path leading west.
You'll be stopped by Hugh. Zinzolin and a Team Plasma Grunt will suddenly show up! Zinzolin will say something to piss Hugh off and he'll begin raging. Select 'Yes' when asked and you'll team up with Hugh to take these guys down. It's a fight in the streets!
Double Battle: Team Plasma Zinzolin and Team Plasma Grunt
Hugh's party, if Snivy was your starter
Hugh's party, if Tepig was your starter
Hugh's party, if Oshawott was your starter
After wasting them, they'll promptly leave and Hugh will chase after them. Might be a good idea to go back and heal your Pokemon, before following after them too. You'll head straight into Route 12.