Your objective is to talk to Dr. Earnhardt, who is located quite a ways west of Amanaki Town. Open your map and fast travel to Amanaki, then start going west. It is in your best interests to activate the Radio Tower on the way, this way the map will be revealed.
Earnhardt’s house is located on a big hill. When you get close be careful of wild Leopards roaming about, as they can kill you quite quickly if they wanted to. You will want to come from the north side in order to get to the doctor’s house. Feel free to explore once you get there, but go inside the building in the back for a cutscene.
Go inside the house and upstairs to see your friend, who is ill and will need some mushrooms from a cave to the “west.” Despite what the doctor said, go east to the marker, using the zipline as a shortcut, then swim around the rocks and down into the water to find a cave. Be quick and swim up and into the cave.
Once you’re in the cave, follow the path until you get to a big drop, so take a leap of faith into the waters below. Keep following the path, climbing up vines and stuff, until you encounter some mushrooms. It’s time for some trippy stuff to happen. As you keep going, you will start hallucinating more and more, until you finally break down and appear as if you’re not in the cave anymore.
When you see Earnhardt’s house, you’re nearing the end. Keep going forward until you finally recover from the hallucination. Go and grab the mushrooms ahead of you, then exit the cave and start going back to Earnhardt’s house. Once you get back to the house, talk to Earnhardt, then go upstairs and talk to Daisy. Some cutscenes will occur, then the next mission will begin.
Return to the doctor with the cave mushrooms.