There is a hidden count in the game called "Zero Points" that increases when you achieve certain Code Geass-related goals. If you have 5 or more Zero Points by the time you finish Stage 37, Roger and Banjou will save Shirley from being killed by Rolo, and then after Stage 49, you will be given the option to either trust or distrust Zero. Choose "Trust Zero" (the bottom option) to acess this secret route that gives a good ending to the Code Geass storyline.