If a metallic sheet of negligible thickness is inserted b/w the two plates of a capacitor, what will be its affect on its capacitance?

If a metalic( conducting) sheet is inserted between the plates of the conductors ,  any charge  can not be stored on the platesbecause  in this  condition as the medium between the plates is conducting so as you try to store some charge on the plates the free charges of the intervening medium will move at the oppsite polarity plates and neutrlise that excess charge that you want to give it.