This is a combination lock puzzle for a door to a vault, prison cell, or something similar. The heroes find a door with a mechanism that has four dials, each of which has a bar that extends from it into a center wheel. The center wheel won’t turn while any bar remains locked into it. The key is to turn the dials to specific settings, but the setting of one dial can affect other dials.

All dials start locked in the 12 o’clock position. All dials turn clockwise and counterclockwise but will only stay at four positions: 3, 6, 9, and 12 o’clock. There is no exposed mechanism that can be picked, but you might allow a successful lockpicking check to provide a hint from “feel”.

• Turning A to the 9 o’clock position opens it, but turning it locks C, unless C is in the 3 o’clock position.
• Turning B to the 3 o’clock position opens it, but turning it locks D, unless D is in the 6 o’clock position.
• C will not move if B is in the 12 o’clock position. Turning C to the 3, 6, or 9 o’clock position opens it but locks A.
• D will not move if A is in the 12 o’clock position. Turning D to the 3, 6, or 9 o’clock position opens it but locks B.

A hint might be that if the heroes open B but turn D, which locks B, they need to turn B to some other position and then turn it back to 3 o’clock to open it again.

Solution

1. Turn A to the 3 or 6 o’clock position and B to the 6 or 9 o’clock position. They remain locked.
2. Turn C to the 3 o’clock position & D to the 6 o’clock position. They open.
3. Turn A to the 9 o’clock position & B to the 3 o’clock position. They open without locking C or D.

With all four dials unlocked, the center wheel is free to move and open the door.

Added Trickiness

To avoid your heroes merely destroying the mechanism or smashing thru the door and getting inside, you can say that the door the lock is set into is a dummy (either a stone wall or a tiny, empty vault). Opening the lock properly actually opens a secret door elsewhere in the room (maybe even the ceiling or high on the wall).

Even if the heroes detect this secret door, it can only be opened by working the puzzle lock. (The door can be shifted slightly, which jiggles the puzzle dials, making it clear they are connected).

Fidget Puzzle Version

If you prefer a simpler solution, just say moving dials doesn’t affect other dials. Then the heroes can just fidget with the dials until they open:

• A opens in the 9 o’clock position.
• B and C open in the 3 o’clock position.
• D opens in the 6 o’clock position.

This simplicity may be helpful for players who don’t like puzzles, who are tired after a long session, or who are overly cautious that turning the dials might trigger a trap.