Joe Christie was a Detective Sergeant with the San Francisco Police Department, and was Adrian Monk's last partner before Monk was discharged. Soon after Monk was discharged, he was accused of stealing cocaine confiscated from a drug dealer, causing the deaths of two uniformed officers who were later killed by the dealer. He was dismissed in disgrace. In actuality, he never stole the cocaine. It was the evidence clerk, Clara Toplyn, who stole the cocaine, and smuggled it out via a police auction by customizing a bike that was intended for a girl to become a boys bike to smuggle it out, and then promptly retired due to the drug dealer bribing her with $750,000.
He later found work as the chief of security at a Mega Mart location, where he called in the police to investigate the death of an employee, Edna Carruthers. Christie was still a detective at heart, but the other police were inclined to give him the brush-off. However, Monk noticed evidence that convinced him Christie might be right, and the two men began to work together again. When Christie saved Monk from a murder attempt, Monk decided to give Christie the benefit of the doubt, and reexamined the evidence of his theft.
Monk proved Christie innocent, and together they caught the killer at the Mega Mart. Christie was reinstated to the Department, giving grateful thanks to Monk, and calling dibs on being his partner when Monk's own reinstatement came.
Joe's actor, Enrico Colantoni, famously played Keith Mars in Veronica Mars. Like Joe, Keith is a disgraced law enforcement officer (sheriff), forced out for for the wrong reason (accusing one of the town's oligarchs of withholding information about a murder) and now works in a law enforcement adjacent profession (private investigator). In the third season episode of Veronica Mars, an undercover Keith gives the pseudonym "Adrian Monk".