Abstract: A cylinder lock with a shell defining a plug cavity, a plurality of locking shell tumblerways, and a control shell tumblerway. A plug is rotatable within the plug cavity and defines a plurality of locking plug tumblerways alignable with the locking shell tumblerways, a control plug tumblerway alignable with the control shell tumblerway and a keyway for receiving keys. Positioned within each of the locking plug tumblerways is a locking tumbler that is biased into an active position wherein it projects into one of the locking shell tumblerways and movable into an inactive position out of the tumblerway by a proper key in the keyway. The locking tumblers prevent rotation of the plug when in their active positions and allow rotation thereof when in their inactive positions. Retained by the control plug tumblerway is a control tumbler biased in a latched position and movable into a release position by a control key in the keyway.