A cube $ABCDEFGH$ with edge length $6$ has bottom face $ABCD$ and top face $EFGH$, with $E$ above $A$, $F$ above $B$, $G$ above $C$, and $H$ above $D$. A plane through $A$, $C$, and the midpoint of $\overline{FG}$ cuts the cube into two pieces. What is the volume of the piece that contains vertex $B$?