The Bat, by author Jo Nesbo, follows the story of Detective Harry Hole, who travels from Norway to Sydney, Australia, to help investigate the brutal rape and murder of Inger Holter, a young Norwegian woman. Assisted by Andrew Kensington and other Australian police officers, Harry soon concludes that Inger's death is part of a series of unsolved murders, leading to a number of suspects and shocking developments.

The book opens with Harry Hole's arrival in Sydney to assist the local police with their investigation of a young, Norwegian blonde named Inger Holter. He is met by Andrew Kensington, an Aboriginal detective, who fills him in on the history of Australia's indigenous people as they proceed with the investigation. Early on, Andrew introduces Harry to two friends -- flamboyant cross-dressing clown Otto Rechtnagel and Aboriginal boxer Robin Toowoomba -- but at first the two men seem to have no relation to the case. Harry also meets Birgitta Enquist, a beautiful Swedish girl who tends the bar at the restaurant where Inger worked. They begin dating.

Finding a letter Inger wrote to a friend, Harry and Andrew identify her boyfriend, drug dealer Evans White, as their primary suspect. However, his girlfriend insists he was with her at the time of the murder. As the investigation continues, the detectives begin to realize that Inger's death fits a pattern involving the unsolved murders of other young, blonde women. They decide the crimes are the work of a serial killer. They question Inger's landlord, but he has a strong alibi and is quickly eliminated as a suspect. Harry also gets help from a prostitute named Sandra and others who live on the fringe of Sydney's gay and drug culture.

Eventually, Harry begins to realize that Andrew seems to know who the killer is and is trying to lead him to the same conclusion. After Andrew is beaten up and hospitalized, Harry confronts him, saying he believes Otto Rechtnagel is the murderer and will be arrested immediately, and he ignores Andrew's insistence that he is wrong. Harry and the other officers attend a performance of Otto's circus, but when they go backstage to arrest him, they find that he has been brutally murdered. When they learn that Andrew has escaped from the hospital, he becomes a suspect, a concept that strengthens when he is found hanging in Otto's apartment, apparently a victim of suicide.

Andrew's death causes Harry, an alcoholic, to begin drinking again, but he continues to investigate and soon learns that Andrew was a heroin addict. He eventually is able to prove that Andrew was murdered and the evidence points to Otto's secret lover, whose identity is known to no one. Evans White again emerges as a suspect and Harry asks Birgitta to help the police entrap him, but the plan goes horribly awry and Birgitta disappears. Harry soon realizes that Toowoomba is the murderer, but by then he has killed Birgitta. Harry tracks the boxer to the Sydney Aquarium, where he shoots him, causing him to fall into a tank and he is devoured by a great white shark. Afterward, Harry gets a rose tattoo as a tribute to Birgitta and finds an escape in skydiving.