Walkthrough: After defeating Acerola's Island Trial and returning to the Aether House with her and Hau, you will find Team Skull's Plumeria and two Team Skull Grunts blocking you from entering. You will actually interrupt what they are doing, with Plumeria stating she wasn't expecting you back so soon. You will then go into battle with Plumeria, while Hau and Acerola battle the grunts.
Team Skull Admin Plumeria - Level 37 Golbat, Level 38 Salazzle
After defeating them and they making their departure, you will quickly learn that Team Skull stole one of the Preschoolers' Yungoos and took it to their base in Po Town. They will have also left a message for your character to come to Po Town alone without backup to meet with their boss, if you ever want to see the Yungoos again. With no other choice, you begin your journey to Po Town by first heading on Route 15.