Obtained: Speak with Trunks in Capsule Corp. during the post game as Vegeta. Make sure that Trunks, Goten and Goku are not in your party.
Objective: Find out what Trunks is hiding
Head to Capsule Corp. and speak with Trunks who can be found near Bulma’s lab. When you get done talking with Trunks, exit Capsule Corp. and head east of West City to run into Goku. Approaching him will lead to a cutscene resulting in a battle with a powerful enemy. Defeat the enemy to complete the story.