Lil' Slugger is an enigmatic figure and the main antagonist of the anime Paranoia Agent. He is depicted as a dark persona, a grinning, baseball cap-wearing juvenile assailant who travels on golden inline skates and assaults people with a gold-colored metal baseball bat that is curiously bent. As a result, he is also the cause of a social dilemma within Musashino, Tokyo.
The series relates the lives of several of Lil' Slugger's victims, as well as two detectives, Keiichi Ikari and Mitsuhiro Maniwa, who are entrusted with the task to apprehend the criminal. Tsukiko Sagi, a shy character designer in Japan and creator of the pink dog Maromi, is pressured into creating a new successful character. She is also one of Lil' Slugger's first victims and is the catalyst who jump-starts the events that unfold throughout the series. The two detectives respond by interviewing Tsukiko at a local hospital just a day after the event, but suspect that she is lying about the attack; that is until they receive word of a second victim. The younger detective (Maniwa) sympathizes, but the older one (Ikari) suspects her of lying. The media preys on the story, and a sleazy reporter called Kawazu Akio tails Tsukiko in an attempt to get further details. He is then attacked himself.
As the attacker persists, however, it is revealed that the attacks are not committed randomly, as Lil' Slugger seems to be targeting particularly those who are undergoing crisis and, despite his violent actions, the events seem to somewhat improve the victim's lives in the aftermath. Therefore, whether he is entirely villainous is certainly debatable. Maniwa becomes convinced that Lil' Slugger is a supernatural force, driven to rescue the desperate from their tragedies through violence. He becomes obsessive, broadcasting his warning about Lil' Slugger via shortwave radio and seeking a way to kill the supernatural assailant.
As public fear of Lil' Slugger intensifies, so do his (supposed) attacks, and the line between truth and fiction becomes blurred. At the same time, public anticipation for the launch of the Maromi television series reaches a fanatical high, almost as if the fear of one is feeding (and feeding off) the anticipation for the other. Things come to an end on the night that the Maromi show is set to air. Ikari, now a private security guard, and Maniwa, now a wandering "knight," attempt to battle Lil' Slugger, now an incredibly powerful force. They confront Tsukiko, and she confesses that Maromi was based on a real puppy that Tsukiko had in childhood, whose leash she had one day accidentally dropped, allowing the puppy to run into traffic where it was killed. Instead of taking responsibility for the puppy's death, young Tsukiko invented a story about a bat-wielding, skate-wearing puppy killer, Lil' Slugger's first "attack".
Ultimately, Lil' Slugger is a paranormal figment of Tsukiko's guilt and fear, brought inexplicably to life when the adult Tsukiko desperately needed to escape her responsibilities, then fed and subsequently became nurtured by the fear of the populace. In a way, Tsukiko does fulfill her job by creating a character (Lil' Slugger) that becomes just as big a sensation as Maromi. When Tsukiko finally confesses the truth, and in doing so accepts the guilt for the death of Maromi, Lil' Slugger is defeated.
Taira Yuichi, an intelligent and athletic 6th grader, is ridiculed at school due to suspension of being Lil' Slugger, considering he wears a similar pair of golden roller skates and a baseball cap. His private tutor, Harumi Chono, and whom he is emotionally close to, tries to help Yuichi overcome the pressure. At school, Ushiyama Shougo, a plump and less-athletic individual, is running against Yuichi for the position of student council president and also begins to steal the popularity that Yuichi once had among his fellow students. Yuichi begins to suspect Shougo. Yuichi then aggressively accuses Shougo, but as it turns out, someone photographs it and manages to text it to the whole class. Shougo openly defends Yuichi, despite his embarrassment.
Meanwhile, the detectives continue to question Tsukiko, who is now witness to two attacks. They also question Yuuichi. On his birthday, only his mother and tutor turn up. On the streets, Shougo comforts Yuuichi, who just gets angrier, wishing that Shougo would be the next victim, which he promptly is. Eventually, Yuichi loses sanity and detaches himself from the rest of society by remaining in his room, before finally having egopathic delusions of regaining his fame before becoming a victim of Lil' Slugger himself.
Chouno Harumi, Yuichi's tutor, also undergoes a crisis in the form of dissociative identity disorder, where she possesses a sinister alter-ego known as "Maria", a prostitute by nightfall, whom she attempts to defy. Her two personalities leave answering machine messages for each other. Harumi becomes increasingly desperate when a colleague at Jiai University (where she works as a research assistant), proposes marriage. She accepts but is terrified of being discovered as Maria. She attempts to throw Maria's work-clothes away but changes her personality just as she arrives at the dump. Her psychiatrist insists she must tell her fiance. While visiting Yuichi in the hospital, Yuichi remarks that Harumi needs a doctor more than him. Later that night, her personalities clash, as they struggle to choose sides, while Harumi tries to regain sanity. She then appears with white face paint before finally being struck down as Lil' Slugger's fourth victim.