Patrick Fitz was ostracized as a child for his rat-like appearance. He fled to the sewers of Gotham City and befriended the large mutant rats which inhabited it. He kidnapped other teens trying to make friends, but things turned deadly to those who rejected his friendship.
Fitz was left for dead when his dwelling in the sewers exploded but as it turns out he survived with third degree burns and his left eye was now blind. His body was scarred which made him look even more rat-like. A few weeks had passed since Dana's encounter with Patrick in the sewers but she still had not gotten over the fear of being attacked by rats. During a lesson in Dr. Pritchard's science class, Dana begins to freak out because Nelson had put a white lab rat in Dana's hair. She screams her way out of the classroom and into the hallway while imagining those giant sewer rats that kept her prisoner, were attacking her now. Terry calms her down and removes the rat.
When Terry and Dana head back to the classroom, they are shocked to discover that the entire class has come under attack by rats. The classroom is screaming for help as the rats bite students but suddenly the rats are called away by some unknown source that came from the air ducts. The rats disappear and Terry heads back to the Batcave to get Bruce's input. Bruce tells Terry that he has been noticing numerous accounts of rat infestations being reported and Terry speculates that these rat infestations would be profitable to someone in the extermination business.
On the other side of Gotham, Patrick is working his angle as an expert exterminator. Patrick names his lucrative business "Rat Patrol" and he uses a special whistle that the rats hypnotically respond to. When Patrick's clients see his disfigurements, Patrick explains that the chemicals he uses caused a skin condition but he calms them down by saying that he has refined his methods and his condition isn't contagious. Patrick also has used his profits to do some dental work in order to straighten his teeth. After finishing a job at Carter-Bluestein Industries, Patrick tells his lead rat which he affectionately named Dana, that his next play is at O'Toole's Secure Storage. Bruce alerts Terry that there is activity at the warehouse district.
Unfortunately, Terry arrives at O'Toole's Secure Storage one minute too late. Tim McDougal, the overnight security guard explains to Gotham PD that he was distracted by rats in the security office while Patrick looted the place. Terry observes the crime scene thoroughly and discovers that the wires which is connected to silent alarm, have been chewed through. Terry figures that these rats could only have been trained to be this intelligent by Patrick the Ratboy and Patrick's endgame will eventually be Dana.
Terry follows a pack of rats that were just leaving Dana's home to a mansion nearby Wayne Manor. Bruce sends Terry the information as to who the house is registered to. Terry learns from the registry that Ratboy's full name is Patrick Fitz and he used his profits from his so called exterminating business to buy a mansion in order to impress Dana. Terry continues to follow the trail of rats to the wine cellar of the mansion where the rats immediately head down a iron grate in the cellar floor. Terry removes the grate, drops down and falls right into the sewers.
Terry is greeted by Patrick and his horde of rats. Terry defeats Patrick and his horde by using his batarangs to break the sewage pipes. The sewage water begins to flood the entire area and like rats in a sinking ship, Patrick's affinitive family desert him for their fear of drowning. Terry tries to save Patrick from the flooding waters but Patrick refuses to let Dana the rat drown so he dives in after her. Terry and Dana Tan know that this isn't the last they've heard of Patrick Fitz.