The obvious game-turning play occurred as Felipe fucking Lopez launched a shot into the right field bleachers for the eventual winning run in the 11th inning, but for me the game-changing play occurred in the 6th inning.
Albert Pujols had just tied the game in the top half of the inning, giving the Cardinals new life after watching their ace give up the lead in the bottom of the 5th. Now they put runners on the corners after one out singles by Byrd and Soriano. For the third inning in a row, the Cubs were threatening to score on Chris Carpenter and deal a psychological blow to St. Louis, which, if nothing else, is fun in and of itself.
Then Geovany Soto came up:
Boom. Double play. That was the end of the threat and Carpenter walked away with a no-decision and a quality start. Then Felipe fucking Lopez got to be a hero later on after nobody else scored in the next four innings. It's a Way of Life.