Obtained: Speak with the Krillin as Gohan at the Kame House in the Southeast Islands Area during Main Story Intermission 3 after completing the An Awkward Pair sub story.
Objective: Help Krillin with his problem
Travel to Kame House and speak with Krillin to trigger a cutscene. When it ends, travel to Papaya Town then make your way to the island in the northern part of the map following the sub story marker. This will bring you to a cave. Approach the cave to start a battle with some Skull Robos. Defeat them to complete the story.