The Passing takes place before Dark Carnival in L4D2, right after Dead Center.
The chapter starts with random chatter between characters of part 1 and 2. You'll see either Francis or Zoey on the bridge, but they don't always say the same thing. Zoey! We miss your slow witted jokes!
The game uses the aging Source engine, modified throughout the years with new effects to keep it current. Outdoor scenes look great but indoor scenes suffer.
This is a sample of the rooms you can find in this level. It's just well, ugly. It looks like a quake mod. There is nothing in this room that couldn't have been reproduced in 1998.
Outdoor scenes fare much better. The little touches with fog/rain/light gives off a mood that is missing when you are indoors. Things are not as squarish, light is uneven, objects are not as easy to make out.
The main event of this chapter is the wedding (Think Miss Havisham from Great Expectations).
At some point in the level, you have to pass through a small park, where a small wedding was taking place. Unlike other random events in the game, this event is static, you'll always meet the wedding witch at this location. This witch is different from the others in the game in that she is strategically placed in front of the kiosk. She is also wearing a torn wedding dress, unlike the other witches that only have undergarments.
Tomorrow: Chapter two - The Underground