Abstract: An electromechanical locking mechanism provides a plug with a rekeyable primary lock mechanism such as a tumbler stack, an electromechanical operator such as a solenoid or a motor, and an electronic circuit having a memory, or an electronic memory and an electronic logic stage, controlling activitiation and operation of the electromechanical operator, contained entirely within the plug. Insertion of a blade of a key that is properly profiled and bitted to correctly displace the primary lock assembly relative to a cylinder encasing the plug, and application by the key of electrical power, or of electrical power and a correct data signal, to the electronic circuit, will cause activation of the electrical operator and repositioning of a distal member of the operator relative to the cylinder, and thereby enable torque manually applied to the blade of the key to rotate the plug within the cylinder.