In short: Madeleine is in trouble. The quest becomes available only if she opened up her shop and you've completed Reward and Responsibility.
Difficulty level: Easy.
To begin the quest, go to Madeleine's shop (the abandoned house). You'll meet Katlyn there. Pick up the daggers from the table.
Then, go to the inn to talk to the owner and the waitress.
You'll be pointed at the aqueduct. On your way you may be stopped by guards - answer that you haven't seen Madeleine. She'll show up when you step on the bridge above the slums. Go trough the floor drain.
Look for Madeleine outside - she's behind the rock. Hand her the daggers. When guards show up, you'll have a choice: betray or cover Madeleine. Your further interaction with her depend on it.
If you save Madeleine, she'll ask for money to cover travel expenses. The quest should be completed even if you don't have the necessary sum (50,000).