Knockturn Alley, a play on
nocturnally, is a street off of Diagon Alley in J. K. Rowling's
Harry Potter series. Knockturn Alley is where all the shady stores are located. It comes up several times in the storyline, such as in the
Goblet of Fire, where Harry tries to use
floo powder for the first time. He accidently says "diagonally" instead of
Diagon Alley, and ends up popping out of the fireplace of a store that sells lots of dark artifacts. Since luck is on Harry's side (most of the time), he hears some interesting information concerning the Malfoys, which he then repeats to
Arthur Weasley, who eventually sends a team over to search for nasty magic items on Malfoy's estate.
As Harry tries to get out of Knockturn Alley, he is accosted by several people until he is rescued by Hagrid, who notes that people would think Harry is up to no good if they saw him skulking about. When Harry asks Hagrid what he was doing in Knockturn Alley, Hagrid says he was getting flesh-eating slug repellant, since they were ruining all the school's cabbages.
There is an interesting map of both Diagon and Knockturn Alley floating around the Internet, in theory from Jo's notes. It has some funny store names, and it can help a reader to visualize the area.