A person kills a taxi driver, cutting off the driver's shirt and removing the shirt. The killer sends a letter to an assumed Head of the Zodiac Task Force announcing the return of the infamous Zodiac. Nash recruits Dale Cutter, the retired former Head of the Zodiac Task Force, to help out. Nash suspects a copycat killer because of the exact duplication of the series of killings. Dale is rusty when he checks out a false lead with Nash. Dale rousts a newly released ex-killer who is a taxidermist, but not the Zodiac.
Meanwhile, Joe Dominguez is undercover as a bicyclist for a messenger service. Joe is on the trail of drug dealers using messenger services to distribute drugs. Lt. Shimamura is up for promotion to Captain. Nick, Nash's father, is forced to move in with his son after being thrown out of the old age home once again. Cassidy, Nash's daughter, plans to throw a secret party at Nash's apartment.
Nick deciphers the Zodiac's latest clue which leads Nash and Dale Cutter to Pier 26. Nash and Dale manage to save a third victim, but the Zodiac narrowly escapes. Dale draws a psychological profile of the copycat Zodiac in a meeting with Nash, Joe, Lt. Shimamura, Harvey, and Evan present. The group deduces that the killer is a person with access to official police photos, a person such a janitor who works in the evidence room. They identify Gary Graham as the copycat Zodiac. Nash and company intercede in Graham's last victim. Graham holds a waitress hostage - Nash diverts Graham's attention as Dale shoots Graham dead.
Cassidy is hung over and sick after her secret party. Nash asks his dad to come live with him. A disappointed Lt. Shimamura does not get his promotion to Captain. Shimamura decides to leave the police force and return to his native Hawaii. Nash says good-bye to Shimamura, and afterwards, the real Zodiac gives Nash a call to taunt him.
For more, see commentary for Episode 16