Tsukiko Sagi, a shy character designer who created the immensely popular pink dog Maromi, finds herself under pressure to repeat her success. As she walks home that night, she is attacked by an elementary school boy on inline skates. Two police detectives, Keiichi Ikari and Mitsuhiro Maniwa, are assigned to the case. They suspect that Tsukiko is lying about the attack, until they receive word of a second victim.

Soon the attacker, dubbed Lil' Slugger (Shōnen Bat in Japanese, meaning "Bat Boy") is blamed for a series of street assaults in Tokyo. None of the victims can recall the boy's face and only three distinct details are left in their memories: golden inline skates, a baseball cap, and the weapon: a bent golden baseball bat. Ikari and Maniwa set out to track down the perpetrator and put an end to his crimes. Their hunt is unsuccessful, however, and the investigation eventually leads to both men losing their positions as police detectives.

As the attacks continue, it is revealed that they are not random. Instead, Lil' Slugger seems to target people in crisis, and the attacks, though violent, lead to some improvement in the life of the victim.