Pedants like to say the total charge on any capacitor is 0. For every positive charge on one plate, there is a negative charge on the other plate. The same thing holds true for a series combination of capacitors.
Why the charge on each of the inner plate doesn't go through the wire in between and make the two plates neutral?
Because the negative charges on one of those plates are attracted by positive charges on the other plate of the same capacitor.
Say you have two capacitors connected like this:
You apply 1 V to node A and ground node B.
Now you have positive charge on the first plate of C1, and negative charge on the second plate. Then positive charge on the first plate of C2 and negative charge on the second plate.
The negative charge from C1's right plate doesn't flow to C2 because it's attracted to the positive charge on C1's left plate more strongly than to the positive charge far away on C2's left plate.