Speak to Darren in the Thieving Guild Cellar, just north of the Lumbridge Furnace.
Darren asks you to pickpocket the dummy to show your skills. After you pickpocket the dummy, Darren tells you that the Thieving Guild is short on funds and it needs to expand. He will tell you about a golden chalice stored in the Lumbridge bank vault. He will send you to Robin, at the Lumbridge castle, to give you more information.
Speak to Robin on the south side of the Lumbridge castle. He will tell you that the owner of the Chalice, a wild-haired old man with a bad temper, has taken it from the Lumbridge bank.
Retrieving the Chalice
Head south to Father Urhney from The Restless Ghost Quest in the Lumbridge swamp. You'll find that you can't distract him enough to get the key from him. Head back and talk to Robin.
Robin suggests that you light a fire outside of his window as a distraction. Head back down to the swamp with some logs and a tinderbox, and light a fire outside of Father Urhney's window. Head into his house, ask him about the chalice, and say "Fire! Fire!". This will distract him enough that you can now pickpocket him and get the key.
Use the key on the Chalice display case to take the Chalice, and head back to Darren in the cellar.