A fail-safe actuator for moving a part has in each case a drive (18, 118) by means of which a first or a second drive train (24, 26) can be moved. The drive trains (24, 26) in each case have their own output shaft (34, 38) and can be actuated independently of one another. An energy storage device is coupled with the second output shaft (38), wherein a holding device selectively holds the energy or releases it from the energy storage device, so that the second output shaft (38) can be moved. A rotary entrainment of the first output shaft (34) ensures that in the event of a failure of the drive (18) this is moved into a specified end position. The two output shafts (34, 38) are set in motion via gear wheels (32, 36) if the drive trains are actuated. An assembly unit consisting of actuator and moved part is also described.