Look at the victim's body. Notice the Pentacle on his chest, and look At Sauniere's Feet, Sauniere's Clothes, Sauniere's Left Foot.
Enter your inventory (Q) and examine the Business Card. Rotate it to find an access code: 454.
Use the Cell Phone and dial 454.
You're in the bathroom. Pick up the Soap from under the mirror. Open the window.
Enter your inventory. Combine the Tracker with the Soap, and then throw it out of the window.
You're back by Sauniere's body. Solve the puzzle by changing the letter order from OH LAME SAINT to THE MONA LISA.
Now you play as Sophie. Move to the end of this room and use the Gate Controls. Sneak past the two guards here (holding Shift) and go to the gallery room ahead. Pick up a metal bar from the floor and use it to sneak attack a guard (man, is that game stupid).
Approach the locker. Move the bottle labeled "BIONE". There's an UV Light there, pick kit up.
Look at the Mona Lisa painting and use the UV Light to read the message written on the glass. Another riddle, an anagram. The message is: DA VINCI PAINTED THOSE THAT KNEW BETWEEN BACCHUS AND URIEL LIES THE CLUE.
Defeat another guard and approach the statue in the center of the Mona Lisa room to get a Saturn Sphere.
Enter the Grand Gallery, using the door to the left. Look at the John the Baptist painting (screen). Notice there's something above it (screen below). You'll get a Priory Ring.
Approach Sauniere's Office to trigger a cut-scene with some cops. Hide behind a pillar (screen) and sneak past them inside. You can also fight them if you like.
Look through the things on the desk. Check inside the drawers - there's a bandage in one of them. A knight on the desk points towards a map on the wall. Use the UV Light on it to find a marking.
Help Sophie get into the Air Vent nearby. You'll be in the Restoration Room. Find a Towel here, a Med Kit (it's in the corner, hidden behind some paintings) and Da Vinci's Aerial Screw. There's a note on the desk about the chemicals. Approach the workbench and mix Chitinase, Dismutase and Catalase. Combine the mixture with the tower, and use it to clean the painting in the center of the room.
Go all the way back to Mona Lisa room and sneak to the left side of the painting. You'll avoid fighting a guard this way. Continue on to the next room, and use the right passage this time, to get past two more guards.
Go to the Nike statue and defeat a guard there. Look at the statue, and then approach the display case on the left (Hand of Nike). Use the Priory Ring on a Copper Engraving and you'll get a Fleur-de-lys Disc.
When you go downstairs, two guards will enter the room. You can fight them openly or sneak attack (there's a useful metal bar nearby).
Go downstairs once more, and turn right. There's a Lead Coin on the railing here (screen).
There are 2 more guards before you. Fight them or sneak, doesn't matter. Go to the exit and you're done here.