Police and the Thief is a side quest in World 1 - Kanterbury Forest (Nightmare). In this quest, Police Goblin requests the player's help to catch a thief.
Completion
To start the quest, the player must first talk to Police Goblin, who will tell the player that she struggles to arrest a thief, due to his great speed.
Due to the thief's speed, it is ill-advised to try to catch the thief by chasing him. The player should instead pick up the Police Goblin and then throw her at the thief, causing the thief to fall down. The thief is then arrested.
The Police Goblin then expresses her gratitude at the player by rewarding the player with the 
Star Piece of 1-5 Deep Ancient Ruin, completing the quest.