Obtained: Speak with Goten at Goku’s House during the main story New Encounters
Objective: Catch 3 Blue Fish with Goten
Head to Goku’s House for a cutscene ending with Gohan and Goten going fishing. When you gain control of Gohan, head to the end of the deck and start fishing until you cat 3 Blue Fish. After getting all 3, speak with Goten to have a quick sparring match. Defeat him to complete the story.