Nash and Joe have to deal with some nasty ex-military types who have been killing drug dealers and taking their money. Joe's nervous about his baby being born. Nash finds an old letter his ex-wife wrote him dated the day of their divorce.

Nash's old partner is back in town, but trouble follows him; meanwhile, Joe has a money making scheme involving Nick's horse.

Nash's former SIU rival is now a bounty hunter in pursuit of the same bad guy as Nash and Joe. Meanwhile, Joe fears repercussions from Inger after losing his wedding ring.

Nash is attracted to a determined female detective from Chicago who's in relentless pursuit of a murderous fugitive; Joe calls a hypnotist for a melancholy racehorse.

Nash's old partner enlists his help after his daughter is kidnapped by an armed robber. Nash and Lisa learn that Cassidy has done something less than savory.

When Cassidy is shot by a stray bullet at school, Nash and Joe hunt down a dealer who has been selling guns to teenagers.

Nash pursues serial sex offenders who have targeted his sister Stacy and, in the process, meets her new girlfriend---who's a former flame of his.

Nash both arrests and assists a law breaking computer hacker whose wife has been kidnapped. Meanwhile, Nash and Whitney finally consummate their relationship.

Crooks impersonate the Special Investigations Unit to front their activities; Nash and Joe pose as a gay couple to retrieve a Super Bowl ring.

Joe goes under cover as a prisoner to find out what is happening with a drug cartel leader who has had a friend of Nash and Joe's murdered and is up to more. Stacey tries to get Nick's Will in order with her, Nick and Nash.

Nash and Joe realize that there is a leak in the prosecution team as they escort a key witness who has joined the Witness Protection Program to San Francisco, while Cassidy decides to go out with J.J.

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 investigate a Chinese student who has a vendetta.

Nash and Joe offer freedom to gangland money launderer Carlos Casas if he will testify against others.

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.

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 and his team take the subway home from a bachelor party and are held hostage after walking into the midst of a robbery. Meanwhile, Lisa takes Cassidy to the emergency room when her daughter complains of severe pains but triage is a nightmare.

Nash and Joe are on the trail of two brothers, heirs to a big Mafia family, which is complicated by the fact that Kelly used to be involved with one of them.

Nash tracks a hit man who was his childhood friend; Evan falls for a woman he sees while on a stakeout.

Evan is wounded in a shoot out with an elusive crew of thieves; Dominguez and Nash provide PI services for a high priced ballplayer to find his low priced car.

A convicted anti-Government militia leader leads an escape from a prison bus; Lisa considers leaving San Francisco.

Joe is given only three days to live after he is exposed to a very dangerous chemical when the SIU team investigate the theft of some highly potent drugs.

An Internal Affairs officer declares that Nash's shooting of a lowlife suspected of murder is a revenge killing, and Nash is suspended.