After capturing Sokolov in The Royal Physician, you are tasked to interrogate him in order to find out the identity of the mistress of the Lord Regent. When it’s your turn to persuade him, choose the option to loose the rats onto him and he’ll sing like a canary.
He informs you that he only knows her surname ‘Boyle’, but he was supposed to be introduced to her at a masked ball that night. You will be given a mission to infiltrate the ball in order to find out which Lady Boyle is the one we need to get rid of.
Right after the interrogation you will be approached by Lord Pendleton who will give you a note to be handed to Lord Shaw. He will be the one in the wolf mask at the party. Now go find Samuel and hitch a ride to the Ball. Remember to stock up beforehand.
Head up the stairs on the other side of the canal and proceed forward until you encounter two guards having a conversation. Cross the bridge nearby and you will see a locked gate.
Look around underneath it and you should see a small opening to the other side blocked by some wooden planks. Use your melee attacks to destroy the planks and gain access.
Once you are in a neutral zone, you will see three people having a conversation a bit ahead of you. One of them will lose their party invitation, pick it up and keep it. Nearby, there will be a guard behind a window, show him the invitation, and he’ll open the doors for you. Proceed inside and follow the way point indoors.
If you want to find Lord Shaw, proceed to the Boyle Gardens. You’ll see him talking to two guards. After delivering the note, follow Shaw, and you’ll find out that Pendleton has sent you as his stand-in for a cowboy duel. After killing Shaw, loot him and head back inside to talk to the party guests.
All the Lady Boyles at the party are wearing the same kind of dress and masks but in different colors. One is wearing red, one black, and the last write. Talk to the guests with unique names (Ramsey and Miss White). Get Miss White a drink and she’ll tell you that the Lady in black is Waverly Boyle and the one in White is Lydia Boyle.
Go talk to each of the three Lady Boyles, and soon after this, Lord Brisby will find you and initiate a conversation. He’ll request private audience with you, so follow him. He reveals that your target is the woman in Black. He’ll also ask you to bring her to him unharmed to the cellar under the kitchen. If you are aiming for clean hands, choose this option.
Either way, go talk to her and tell her that you know her name, ask her if it’s Waverly, then inform her that her life is in danger. Ask her to ‘follow you’ and go to the basement, Kill her or choke her out and deliver her to Lord Brisby.
You’re officially done with the mission so simply follow the white marker while evading guards on the streets. You’ll find that Samuel has moved his boat past the canal gate so it will be a bit more complicated to get to him.
Don’t move across the canal but go up along it (avoiding or choking out the one guard and avoiding the Tallboy) until you get to the gate/dam, now go across the gate using the bridge atop it. There may or may not be a Tall Boy on the other side; he will be a bit annoying but quick use of the Blink will drop you into the canal on the other side of the gate thus avoiding detection.