A mechanical timepiece movement in the form of a modular unit. The movement includes at least one modular unit for performing a particular timepiece function, which is irremovably pre-adjusted by irremovably securing adjustment and/or assembling components included in the functional module after the particular horological function has been adjusted and function checked on the test bench. The pre-adjusted functional module is irremovably secured to a plate included in the movement or included in another pre-adjusted functional module of the movement, or irremovably secured to another pre-adjusted functional module included in the movement.