Nash's accountant commits suicide because he lost all of his clients money. Nash then learns that the man was involved in a pyramid scheme and an old flame is behind it, which makes him even more determined to get his money back.
Nash butts heads with a cop whom Nash thinks is dirty. When he kills him and two other cops, he arrested because there's some personal stuff between them which is believed to be the actual motive for their feud. Bettina is assigned to defend Nash and he brings in a cop from Internal Afffairs, Caitlin Cross to find out the truth. But she's operating under the assumption that Nash is guilty. Nash also claims that he thinks that there was another man at the shoot out, who got away, whom he thinks is their partner.
Nash and Joe are on the trail of a renowned assassin who has come to San Francisco to kill FBI special agent David Katz. Lisa Bridges returns from Paris and announces she's engaged to marry a famous French chef named Henri, but there's a problem; Nash and Lisa's divorce was never finalized-they're still married. The new S.I.U. is introduced.
Nash and Caitlin go undercover as swingers to bust a male-female team who prey on wealthy swinging couples. Joe and Boz help Nick recover the corpse of an old friend to whom Nick made a war-time promise.
Nash becomes the objective of mercenaries controlled by a Singapore billionaire and his American counterpart, locked in battle to capture each other's most prized possessions. Bettina wants Joe and Evan to investigate his fiancee's alleged affair.
Nash pursues a deadly arsonist who torches a church and the only witness is Angel. Caitlin investigates Evan on a police brutality charge, probing into his past. Nash's friends and family plan a surprise birthday for him.
Nash and Joe have to babysit Mickey Tripp, a motormouth shockjock, after a body armor-wearing assassin tries to kill him. Their search for the assassin is compounded by the fact that Nick has hired them on an off-duty case to find his softball team's star pitcher - a Salvadoran defector named Chus, who's on the run from the INS. Caitlin babysits her roommate's kids.
Nash finds himself defending Caitlin when she's the main suspect for a murdered CIA agent connected to her mysterious past. Joe begins working out with the trainer from hell. Cassidy and Nick enter a tango dance contest.
Nash is accidentally shot in the butt by Caitlin, leaving Rick Bettina in charge. Rick is duped by phony FBI agents attempting to swindle half a million in sting money. Cassidy is stalked by a violent classmate.
Harvey discovers his counterculture pal is involved in a murder plot to benefit from a proposed new stadium site. Joe clears his "daughter" accused of stealing jewelry at her department store job. Nash helps Caitlin get into an exclusive VIP party for an artist.
Harvey tracks down two teenage boys who are deadly snipers. Joe has a rat problem in Nash's house, causing a dispute between landlord and tenant. Evan proposes marriage to Cassidy; Cassidy is involved in a deadly auto accident. A case of mistaken identity causes Nash to think Cassidy has been killed.
Nash and company, along with marginal cop Jake Cage, track down pushers of a deadly steroid which makes body builders go hyper-aggressive. Cassidy is haunted by the death of her friend from the auto accident.
Nash and the SIU secretly investigate a plot involving the missing married Governor of California who has fallen in love with a former prostitute. Joe gets stuck with a painting while bidding for someone else at his daughter's pre-school charity auction.
The Bayside Killer resurfaces after a four-year absence, while Nash finally unravels Angel's identity. Harvey's ex-wife Bonnie stops by for a visit. Joe and Caitlin coach rival girl's soccer teams, with Caitlin's new boyfriend as her assistant coach.
Nash and company discover a gang of female bank robbers who plan to break their husbands out of jail. Nick schemes to move into a formerly all-women retirement home. Caitlin readies her negative report on the SIU, much to Nash's consternation. Guest star Jo Dee Messina.
The SIU is disbanded by MCD: Nash and Joe are assigned to Park Detail, Harvey to Homicide, Evan to Traffic, and Caitlin to Vice. Still, Nash manages to solve a murder involving the Head of MCD's daughter and a drug dealer. Evan and Cassidy announce a wedding date; best man Harvey throws a bachelor party, but finds out the stripper is Anna. Nash tells Caitlin he loves her, but she accepts a job in Boston.
Nash and company, along with Jake Cage, catch three escaped convicts to save the SIU. Harvey has fathered a child with Anna; Evan cheats on Cassidy; Nash and Caitlin finally consummate their relationship.