Joe Friday is working the Forgery Department. He and his partner respond to a small business, where a salesman had a bad check passed to him. He had complete Id and documentation from the crook when he cashed it. It seems to be the work of a ring of forgers.
Friday gets a tip from a woman who is a released convict who wants to do a good deed, and stay clear of the forgery biz. She is trustworthy enough for Friday to use in a trap.
On the stakeout, things start to fall apart. With more leg work, the forgers press is uncovered, and stopped.