While you're in the Thieves' Guild help yourself to the willow blackjack and lockpick, if you don't have them already. Dodgy Derek sells them in the western-most building. Look for the general store symbol to find him.
The next bit you have to do quickly! Read it before you proceed.
Click on the ornate cuckoo clock to wind it and run back into the main room with the guard. You must stand next to the counter, not by the doorway to the cuckoo clock room.
As soon as the Registrar is in the cuckoo clock room, lure the guard, knock him out, and then steal from the Guard to obtain the vault key.
Run as quickly as you can, behind the desk in the main room and pull the lever on the wall to open the trap door. Quickly climb down the trapdoor to enter the vault room.
If the Registrar returns to the main room before you enter the basement you will have to repeat the entire process. It may take a few tries to succeed.
Vault room[]
When you succeed in getting down the trapdoor, the Registrar will ask Bernard what happened and then lock you in the vault to "stew for a while".
Pick the lock on the gates to enter the area with a large chest. (This may take many attempts, even with a lockpick.) (If it's not working, try using the lockpick on the door)
Check the Registry Strongbox to look for traps (otherwise you get hit for up to 3300 life points).
If players take the vial of stench (a) from the quest back to Chief Thief Robin, he re-enchants it so players can sell stolen goods to shopkeepers as long as they have the potion in their inventory
This article on a miniquest has an associated dialogue page. Click here for a transcript of possible conversations encountered throughout the miniquest.
Trivia[]
If you talk to the guard before winding the cuckoo clock, he will say that the stench is vile. This is a play on vile being a homophone of vial, which in turn references the vial of stench.
Completing this caper is a requirement for the Completionist Cape, as a piece of music cannot be unlocked without it.
It is impossible to enter the Guild Registry after stealing the bank notes, as the player doesn't want to "tempt fate by going in there."