After you star in a film in Pokéstar Studios, go to the southeast of Virbank City, toward the dock. Team Plasma is there, causing a ruckus!
Fight Team Plasma
Roxie and Hugh are facing off with the Team Plasma Grunts. After some angry words are exchanged, one of the grunts comes up to you. Talk to him and say yes to fight him.
The Team Plasma Grunt that you fight has a level 14 Patrat.
When you win, the Team Plasma Grunts will run away. Roxie asks you to check Route 20 to get rid of any Team Plasma Grunts hiding there. She gives you HM01 Cut.
Get Rid of Team Plasma in Route 20
Head west through Route 20. Hugh is on the bridge. Don't cross the bridge. Go south from where Hugh is standing and you will find the Team Plasma Grunt who is hiding there. She has a Level 14 Purrloin.
When you defeat her, she runs away and says something that makes it seem like Team Plasma is going around by boat and might be going to Castelia City.
Hugh tells you to go back to Virbank and look for someone who knows about ships.
Go to Castelia City
Go back to the area of Virbank City where you fought the Team Plasma Grunts. You find Roxie complimenting her father on the movie, but he admits that he was too excited to do a good job.
Now you can go to Castelia City! Follow Pop Roxie into the ship building.
Talk to Hugh and say that you want to go to Castelia City. You'll go there right away.