Venetica

First you must choose to join the Net of the Mask as your guild of choice. Their HQ is in the Inner City, but you can join (sort of) before you get access to the Inner City. While in the Outer City, go to the Inner City entrance to get a cutscene of an argument between Aeris and some Persian guards. Afterward, speak to her and tell her you wish to join her guild. She'll tell you to met with her in the Inner City. Once you gain access to the Inner City, go to the Net of the Mask HQ (just down a little and on the left from the entrance) and speak to her again. She'll tell you to meet her in the library (sheesh!) so activate your Passage ability and follow her through the Twilight Portal. Speak to Aeris again and she will (finally!) let you join. Speak to her yet again (this is like pulling teeth) and she'll give you your first quest.

Quest 1
Note: Make sure you have a hammer/axe weapon with you for the wooden doors.

Head outside and back through the gate to the Outer City. Go into the house that's right there next to the Nightwatch Guild. Head downstairs to the cellar and grab Victor's diary from the chest. Take the diary back to Aeris and pick up the next quest.

Quest 2
You will not be able to complete this quest until you have the Whisper ability, which you will not receive until you gain access to the Arsenal district. (You'll get the Whisper ability when you clear out the demons from the chapel in the Arsenal district for Brother Johan). Once you have it, head all the way back to the Southern Shore, activate the quest in your journal, and follow the marker to a skeleton lying near the area where Hengfisk used to hang out. Use your Passage ability and talk to the dead guy to get the letter from him. Take it back to Aeris, who will question you. Tell her everything you know. Get the next quest from her.

Quest 3
Head out to the Forest Hut (way back on the San Pasquale map where you first met your lockpicking buddies) and go inside. Find the skelton, use your Passage Ability, and talk to the dead guy. After laying some lengthy prose on you, he'll turn over his notes and you can head back upstairs and grab the goods. Head back to HQ, talk to Aeris, and get your next quest.

Quest 4
Go back inside the library there in the HQ using your Passage ability to access the Twilight Portal. Once inside, go upstairs and activate the book puzzle in the sequence 1-4-3-1. A bookshelf will open up leading to a passage. Follow it to a Gateway Idol where Benedict will show up and lay the Ring of Ghosts on you (regenerates MP).

You're done!