You’ll get the SecretPotion from Cynthia shortly after you defeat Crasher Wake at the Pastoria City Gym.
For the first half of Pokémon Platinum, a group of Psyducks is blocking the path to Celestic Town. The SecretPotion is necessary to remove these Psyducks and open the way to Celestic Town.
Moving the Psyducks on Route 210
For a significant part of the game, a bunch of Psyducks will be barricading a path that leads to the northern section of Route 210. You’ll need to unblock this path in order to reach Celestic Town and obtain HM03 Surf.
The sad truth is that the Psyducks are all afflicted with paralyzing headaches.
And you’ll need to offer them some relief if you want them to budge.
You can do this with the help of a special medicine called the SecretPotion.
How To Get the SecretPotion
After you’ve defeated Crasher Wake in Pastoria City and won the Fen Badge from him, your panicked Rival will greet you and Wake outside of Pastoria Gym.
Your Rival informs you and Crasher Wake that a Team Galactic henchman has been hanging outside of Pastoria City’s observatory, talking ominously about setting a bomb off inside it.
This enrages Wake, who dashes off with your Rival to confront him.
Head to the observatory at the north edge of town, where your Rival and Crasher Wake will be waiting for you.
But it’s too late—shortly after you arrive, a bomb discharges inside the observatory.
The Galactic Grunt who set it off runs out of the building, talks some trash to the three of you, and dashes away.
As Crasher Wake heads into the observatory to make sure no further damage is done to it, you’re tasked with chasing after the Grunt.
This isn’t much of a “chase,” though. So you just need to head to specific pre-determined spots to catch up with the Grunt.
First, head east toward Pastoria’s entrance gate to find the Grunt just sort of hanging around in front of it. Speak with him and he will run through the gate.
Follow him through the gate to find him standing just on the other side. Speak with him again here.
You’ll need to repeat this chase/talk process a few more times.
He will next head to the beach, to the southeast.
He will then run northeast and pass through the Hotel Grand Lake’s front desk building.
Chase him down to the northeastern corner of the hotel complex.
At that point he will run down the steps and stop just a short distance away. Speak to him one more time.
The Grunt will complain about being too exhausted to keep running. He will then instigate a Pokémon battle with you—but with only a level 31 Croagunk, he’s just as easy as all the other Grunts you’ve fought.
Make quick work of him and the Galactic Grunt will run away for good.
Cynthia will then show up to the scene shortly after you win.
She’ll mention the group of Psyducks blocking the path to Celestic Town, and then she’ll hand you the SecretPotion to help you deal with them.
How To Move the Psyducks
Once you get the SecretPotion from Cynthia, it’s time to head back to the Psyduck blockade.
The Psyducks are by the Café Cabin on Route 210. If you don’t remember where the Café Cabin is, Fly back to Solaceon Town and head north.
Leave Solaceon Town for Route 210. Then use your Bicycle in fourth gear to make it up the muddy slopes on the hills for faster traveling.
Keep heading straight northward to reach the Café Cabin.
You’ll find the miserable group of Psyducks to the left of the Café Cabin.
Approach the Psyducks and interact with one of them. You will be prompted to use the SecretPotion—select Yes.
The SecretPotion will lift the Psyducks’ crippling headaches as if by magic.
They will then clear the area, allowing you to head northwest to Celestic Town.