While coming home from a late night bachelor party, Nash and some of his fellow cops become embroiled in a train heist gone bad on a BART train.

After a nearly 20-year absence, the notorious San Francisco serial killer, ""Zodiac,"" returns to taunt Nash as well as haunt Nash's mentor (who has since retired). But is it the actual Zodiac, or just a lethal copycat? Meanwhile, Joe goes undercover as a bike messenger to nab a free-wheeling drug dealer.

Nash is stuck with an eleven year old delinquent who's the link to busting a drug kingpin. Joe is reunited with his grown son J.J. and Nick wreaks havoc with Nash's attempt to find him a live-in nurse.

Nash and Joe cut a deal with Carlos Casas, a gangland money launderer, offering him a new identity in exchange for his testimony.

It's Christmas Eve, and Nash and his fellow cops are trapped at the SIU as they wrap up a murder investigation and a prostitution case. Joe borrows Nash's 'Cuda to get it detailed and freaks out when the car is stolen; Nash's personal gifts to Lisa and Whitney get mixed-up; personal crises abound as the cops deal with the Christmas holiday.

Nash and Joe have a task of escorting a secret key witness from the Witness Protection Program to San Francisco to testify in an important SIU-related case. They fall under gunfire. Nash realizes there's a leak somewhere on the prosecution's team. He enlists the help of Stacy to protect them until he can solve who it is and get the witness safely to San Francisco. Cassidy is stood up for a date and decides to go out with J.J. instead.

Joe goes undercover inside State Prison to bring down a ruthless Mexican gang leader who put out a hit (from behind bars) on a SFPD undercover cop who had infiltrated the Mexican Mafia. Nash runs the investigation on the outside, hoping to fulfill a promise to the dead man's widow by catching the trigger man. Naturally he'll have to overcome the distraction caused by Nick, putting together his will, who discovers some long-forgotten wealth.

Nash busts computer hacker/robber, Andy, and discovers Andy's committed the crime because his wife has been kidnapped. The crew of crooks want Andy's hacker skills as their ransom. The SIU must work to save the woman and bring down the crew before their daring daytime robbery is successful. Meanwhile, Nash and Whitney finally consummate their relationship.

After Cassidy is hit in the behind by a stray bullet at school, Nash and Joe go on the hunt for the shooter and the dealer who's putting guns into kid's hands. Due to earthquake repairs on his apartment, Nash and Nick have to take up residence with Lisa for a few days. It's here that Nash meets Freddie, the apple of Cassidy's eye, the pain in Nash's behind, and learns that Cassidy's decided not to go to college.

Hot on the trail of a murderous criminal from Chicago, Nash teams up with a KAREN DECKER, a cop from the Windy City who's in every way his equal: smart, competent-and yes, sexy. Sparks fly between them as they track down the visiting villain together -until, that is, Nash learns his newfound partner is not as perfect as she's led him to believe. Joe, meanwhile, hires a pet psychologist to cure what ails Nick's racehorse, Mr. Woody.

Nash's old nemesis from the SIU, Rick Bettina, now a bounty hunter, is back and in competition with Nash as they both try to track down mobster Ray Goetz, who Bettina wants on assault and battery charges in Vegas, and Nash suspects of ""hitting"" a high-level mob boss. Meanwhile, Joe fears repercussions from Inger after losing his wedding ring.

Nash solves murders in Chinatown and helps out an old friend, Cedrick Hawks, a professional gambler who is in San Francisco to clear up a bad debt.

Nash and Joe butt heads with the leader of a team of Urban Commandos who has been hired to protect an inner-city community. While awaiting the birth of the new baby, Joe searches for the family crib that he lent to Nash seventeen years ago.

Nash must stop a sniper terrorizing the city by shooting victims at random. A rat infestation forces Joe to confront his ""landlord,"" Nash, about the problem. Evan proposes to Cassidy, but in the wake of a serious traffic accident, Cassidy is left fighting for her life.

Nash and the SIU must re-open a serial killer's case file when a body is discovered and it appears to be Angel's. Bonnie, Harvey's ex-wife, returns with an interesting proposition. Joe and Caitlin square off as rival soccer coaches when their teams enter the playoffs.

Nash and Joe investigate a missing hockey goalie connected to the Russian Mafia with the help of kidnapper Tony B., who has lost all memory of the crime. Joe visits a fertility clinic when Inger decides she wants another child. Harvey and Evan shoot off the roof of the watch commander's brand new squad car and must get it fixed before he finds out.

An ex-cop is released from jail and seeks revenge on Caitlin who put him there. Nash teams up with Caitlin to capture him. Joe and Inger continue their efforts to become pregnant, discovering how difficult it is. Harvey and Evan go underground as wealthy conventioneers at a ritzy hotel bar to catch a follow-home robber.

Four robbers storm into a check-cashing outlet. They release tear gas grenades and rob the place. Nash, Joe, Harvey, and Evan respond to the silent alarm. Three robbers (who all turn out to be women) dump the cash into a getaway car (driven by another woman) and leave on foot. Harvey and Evan run into the getaway car, but let it go, not knowing that the robbers are women. Nash and Joe confront the fourth robber. Nash shoots him dead through the eye hole in his gas mask. The dead robber is Adrian Beltran; stolen is 100,000 dollars. The MO (modus operandi) is similar to a North Beach robbery crew from four years ago, who are now all in prison. Nash suspects Anthony Warrick, who ratted out the original crew. But Warrick retains heavy-duty lawyer Denise, and Warrick is clean with no connection to the dead man. While Harvey and Evan surveil Warrick, another robbery goes off. A trace of the tear gas grenades leads to Bunker Bob, but he only identifies the dead Adrian Beltran. A study of the

Handcuffed Skip Osaka jumps out the back of the moving 'Cuda. Nash and Joe give chase. Skip runs up a fire escape onto a power line. The line breaks, and Skip falls through a window. Nash and Joe place him in the trunk of the 'Cuda, and bring him to the SIU. A surprised Nash, Joe and Harvey discover the SIU barge is being towed away by a tugboat. The Head of the MCD, Reuben Banks, drives up and informs Nash the SIU has been temporarily shut down. The SIU is disbanded by MCD: Nash and Joe are assigned to Park Detail, Harvey to Homicide, Evan to Traffic, and Caitlin to Vice. At the Civic Center Park, Nash and Joe meet Colonel Culpepper, drug addict/transient, and Phil, the phony Vietnam veteran. The Colonel is shot dead in the park. Harvey and his new partner, Inspector Riley Webber, investigate the murder. Evan and Cassidy have set a wedding date. Harvey is best man, and he plans the bachelor party. Nash and Nick are informed, and Nash gives his blessing. Father Nick gives Cassidy her g

Nash and Joe meet Lyle who tells them the secret location the SIU barge has been towed to. Joe returns Lyle's chihuahua Ginger who has been held hostage in return for the info. A truck deliberately rams into a van containing three prisoners. Nash and Joe arrive at the accident scene, with Nash figuring out that a phony paramedic van has taken the three prisoners in a successful escape attempt. Nash, Joe, Harvey and Evan arrive at the empty and disconnected SIU. The Mayor has granted a one week reprieve pending the Police Commission hearing. If Nash and company catch the three escaped prisoners, the SIU will be saved. Head of MCD Reuben Banks daughter Gina's immunity deal is also dependent upon the return of one of the escaped prisoners - Gina's former boyfriend, Jimmy ""Shank"" Brasher. The other two escapees are Darnell Galloway and Elliot Wong. Gina is kidnapped by Brasher. Jake Cage of Fugitive Recovery is already on the case, beating Nash to interview Mr. Footman, owner of the stolen

Jake Cage and Caitlin go undercover to capture an international fugitive returning home for the wedding of his daughter. Nash and Joe steal a violin for corporate clients, only to have the violin stolen again by one of Joe's detective students. Evan gets involved with a street prostitute.

An embezzler with a twin, along with bumbling Rick Bettina, confound the efforts of Nash and the SIU to recover stolen city funds. Caitlin tosses Nash's baseball out the window after they miss their flight to Jamaica.

Lowlife Frankie Dwyer returns, this time importing Vietnamese women as sex slaves, as Caitlin goes undercover as the seller. Nash has a tooth ache and refers to Caitlin by his first wife's name. Joe is aggressively pursued by bodybuilder Spike; Evan bumps into Cassidy in his job as a parking lot attendant.

Nash goes undercover as a hitman for a drug cartel. Nick's poker buddies plan a bank heist; Inger returns from Sweden with her mother; Evan gets his engagement ring back from Cassidy.

A married woman is murdered by her lover and his wife. Evan is beaten up trying to free his girlfriend/hooker from the clutches of a pimp. Joe takes Evan in, but Evan attempts suicide. Boz Bishop asks for a promotion at the detective agency.