Pokémon Diamond and Pearl/Celestic Town

After getting the SecretPotion from Cynthia, head to Route 210 and cure the psyducks of their headache. Route 210 leads to Celestic Town.

Once you get here, go to where the Galactic grunt and battle him.

If you haven't already, go to Pastoria City to find a Team Galactic grunt at the top of the city. Talk to him and he will run away. Follow him, (you'll run into your Rival at this point, who will want to battle) and keep following the grunt out of the city to Valor Lakefront until he battles you. Cynthia will then appear and present you with the Psyduck Antidote that you'll use to remove the Psyducks on Route 210 next to the house with the MooMoo Milk. You will battle a lot of team Galactic grunts.