The Prison Cells Key is an Inventory Item that can be used in the game.
The Prison Cells Key is a small silver-grey metal key.
The Prison Cells Key can be found hanging from the belt worn by the Thin Black Cap guard.
When Josef is under the table in the Guardroom he should wait for the guard to lean back in his chair. When the guard does so, Josef can push the leg of the chair making the guard fall backwards onto the floor. This will give Josef the opportunity to steal the Bullets from the ceramic plate that is on the table. When the guard gets back up he will notice that there are no bullets to use. The guard will then go to the dartboard and pick some bullets out of the dartboard. When the guard is standing at the dartboard Josef can carefully steal the Prison Cells Key from the guard’s belt.
The Prison Cells Key can be used on the prison cells security panel. There is a security panel on the wall of the Guardroom that controls the doors of the two prison cells. There are two red lights with keyholes beneath, one for each prison cell. If the light is red; it means the cell door is locked shut, if the light is green; the cell door is open. Josef can use the Prison Cells Key to unlock the two prison cells letting the Prison Robots escape. Unfortunately the guard will catch the escaping prisoners and lock them back in their cells.
Note: see Bullets for how to complete the successful prison escape mission.