Speak to Isaac and Danielle near the PokeCenter @ -186 38 413

Gather clues

Speak to Danielle @ -223 38 329

Return to Isaac @ -185 38 411

Go to the lake and speak with Danielle @ -155 38 363

Jump into the lake and find the entrance @ -85 21 315

Follow the path until you fall down a hole

Find and defeat Cosmic Star Grunt @ -166 2 288

Find and defeat the next Cosmic Star Grunt @ -143 7 220

Find and defeat the next Cosmic Star Grunt @ -60 6 181

Find and defeat the next Cosmic Star Grunt @ -52 2 302

Find and defeat the Cosmic Star General @ -56 1 379

Do /spawn and speak to Professor Hemlock inside of his house next to his lab @ 770 46 882

Once you have spoken to Professor Hemlock, you will have completed the quest and will receive these rewards: