Cohle is introduced as a gifted, but deeply troubled, detective from Texas who is transferred from working on regional drug task force to the Louisiana State Police near the end of 1994. After three months with the LSP, in January 1995, he and his partner are tasked with investigating a series of brutal, bizarre murders. A haunted, solitary cynic, Cohle believes that life is meaningless, human beings are merely "sentient meat", and hope is self-delusion. Cohle spends his free time obsessing over every detail of the crime, hoarding evidence in a storage locker and keeping extensive notes in a ledger, which earns him the derisive nickname "The Taxman" among his colleagues.
The series gradually reveals Cohle's backstory. He was born in South Texas but raised in Alaska by his father after his parents divorced. He became a police detective in his 20s. Years before the main story arc, his two-year-old daughter, Sophia, was killed in a car accident, a tragedy that destroyed his marriage. Devastated by the loss, Cohle grew ever more unstable and unpredictable in his work, eventually killing a crystal meth addict who had injected his own child with the drug. His superiors offered him a chance to avoid prison by working as an undercover narcotics detective, which he did for four years; Cohle notes that this was twice as long as most undercover detectives are kept in rotation. During this assignment, Cohle became addicted to drugs, and eventually killed three cartel members in a shootout, while being shot multiple times himself.
During his recovery, he was committed to a mental hospital in Lubbock, Texas. Upon his release, he was offered retirement with full pension, but he declined that offer in favor of transferring to a homicide division. His superiors then transferred him to Louisiana. In Louisiana, he lives alone and has no friends, living only for his work. He tries to remain sober, but he occasionally falls off the wagon because of the stress of his job and his grief for his daughter. Cohle also has synesthesia and suffers from flashbacks from his drug-using years as an undercover cop.
The series takes place in two time periods: 1995–2002, in which Cohle and Hart work together to find the killer; and 2012, when Cohle, who has by now quit the police force and become an alcoholic, submits to an interview with Louisiana State Police Department detectives Maynard Gilbough (Michael Potts) and Thomas Papania (Tory Kittles) regarding the murders. Cohle sees through the detectives, and realizes that they think he is the killer.