Charles is correct, it is on page 16 of the Villains rulebook:
Running out of Cards in the Ally Deck or Adversary Deck wrote:
If either of these decks runs out of cards before
the Plot is completed, you finish the turn, and
then the players have successfully survived the
Plot, but they didn’t defeat the Commander. As a
result, the game is a draw between Good and Evil.
The player with the most Victory Points wins an
individual victory. Be sure to get the Commander
It is worth pointing out that until the end of this last turn, you can still win or lose by defeating the Commander's last tactic or fulfilling the Evil Wins condition, respectively.
Hope that helps