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.

Willie Nelson plays Earl Dobbs, a convict who gets a furlough from San Quentin to help Nash and Joe catch his former crime partner.

Nash and Joe track down a former mob underboss who's murdering San Francisco criminals. Meanwhile, Inger has to go out of town, leaving Nash and Joe with the baby.

Nash, on the verge of closing a case, narrowly avoids getting killed in an explosion, losing a week's worth of memory as a result. With Joe's help, he has to retrace his steps, resolve the case, and deal with the various personal snafus resulting from his memory loss. Michelle joins the SIU.

Cassidy finds herself in the middle of an SIU case when she becomes the victim of a high-climbing jewel thief and Nash begins to close in on the suspect: her new boyfriend. Stacy becomes concerned that Nick is being turned into a lab rat at the university where he has volunteered for a research study.

Sultry criminal and Nash's ex-girlfriend Tamara VanZant resurfaces, this time as a player in a plot to sell stolen plutonium. Joe plans to make a mint by marketing a secret-recipe salsa that's been in his family for generations. Robert Rodriguez stars as Joe's commercial director.

Rick Bettina returns as the head of the S.I.U. Joe is accused of stealing 3 million dollars from a suspect. Harvey has a torrid affair with a woman he once arrested.

Geraldo Rivera comes to the SIU to interview Nash and Joe. Nash arrives at the SIU after having his drink spiked with acid by a biker couple; and there's a ""mole"" in the SIU feeding sensitive information about Nash to Geraldo, which he uses in the interview. Meanwhile, our SIU cops must solve a mystery which begins as a routine shooting at a convenience store.

Nash and Joe investigate several brutal drug-related gangland murders and follow a complicated trail of clues to a surprising suspect -- Bruce Sutcliffe, a multi-millionaire from a well-respected San Francisco family. When Nash discovers that Sutcliffe has been a major drug kingpin for years, he risks his job and his life to expose the magnate's secret life. Meanwhile, Evan and Bettina go undercover in a transgender beauty pageant to trap a blackmailing murderess who pretends to be a man impersonating a woman.

Bettina is accused of killing a high-class call girl. Only Nash can help Rick find out who framed him for murder. Meanwhile, a man masquerading as Joe has become a major romantic figure in town (with Joe's credit cards), much to Inger's chagrin.

Nash must protect Lynette (Suki Kaiser) from ruthless Hong Kong mobsters when she unknowingly becomes involved in an Asian forgery ring. When Lynette's old flame, Brian, mails her a flawless forgery plate for $100 bills, Nash must protect her from the brutal Hong Kong gangsters from whom the plate was stolen. When Brian unexpectedly appears at Lynette's door, their attraction is re-ignited, but he soon becomes the chief suspect in Nash's investigation into the forgery ring, run by an Asian crime syndicate. Meanwhile, Joe's visiting father (Ismael Carlo), a sheriff from New Mexico, drives Joe crazy with his meddling until he uses his own brand of Wild West sleuthing to expose a murderer in Joe's apartment building.

When the ruthless high school dropouts go on the crime spree, Nash's search for the couple leads him into an underworld of heavy metal rock and satanic worship. As the intensity of the crimes increases, Nash must race against time to stop the desperate duo before they murder all the enemies on their ""grudge list."" ; Stacy and Nash meet Nick's long-lost love from the war years; Joe finds Evan and Cassidy in a compromising situation.

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.