Head to F.E.A.S.T. and press 
by the duffel bag on the roof as always to enter the building as Peter Parker. You can speak to Aunt May before walking up the stairs to the second floor, where Martin Li's office can be found. Examine the door to find that it's locked. Some homeless men will speak to Peter upon seeing him, but fortunately, the hall empties out quickly. Zip to the vent on the ceiling and begin to crawl to his office from here. You can examine a vent on the way, giving Peter a view of a room he's never seen before. A short crawl past this are some sparks blocking Peter's way. Aim at the junction box in the distance and shoot an Electric Web at it to continue crawling. There's one more vent along the way giving Peter a view of Aunt May, after which you can take a right turn to finally enter Martin Li's office.
Examine the shrine to Li's parents. The frame holding a photo of them contains a mysterious keyhole. Examine the book by the chessboard. It tells the story of a demon that can only be controlled through balance. Examine the photo on Li's desk (press 
to have Peter pick it up for a closer look). The shrine can be seen in the background, but the photo of Li's parents is absent. Also on the desk, you can press to read a letter from Li to May. Behind the desk is a wall of accolades that you can examine, showing all the good that Li has done for the city. Finally, check the drawer of the desk to find a book with a key hidden inside. Take the key and read the bookmarked journal entry for some insight into Li's actions. You can then return to the shrine and examine the keyhole to unlock a puzzle.
You must rotate the pieces of the puzzle so that they form a yin-yang symbol, in line with the storybook describing a balance to control the demon. Start by rotating the left lever counterclockwise twice, so that the smaller white circle covers the bigger black and white circle. Rotate the right lever counterclockwise four times, so that the white circle is above the black. Lastly, rotate the center lever clockwise twice. Release to unlock the hidden room seen through a vent earlier.
Approach the table to the right to examine the mask and sword, gear utilized by Demons throughout Li's organization. Examine the corkboard at the back of the room to discover Li's obsession with Norman Osborn. Approach the table in the center of the room to examine an eerie photo, a file describing something called Devil's Breath that you can take, and a tape recorder that you can listen to. Back away from the desk to trigger a scene, after which Peter ends up on the ceiling. Shoot the junction box on the wall left of the desk with an Electric Web from here, as well as one on the wall to the right of the entrance; you must shoot both boxes three times each in order to disable the electricity and salvage the evidence. Drop through the vent on the floor and push aside a bulky object blocking the door to exit the secret room. After a scene, Peter puts his suit back on and the mission is concluded.