Mac and Stella investigate a series of robberies targeting some of Manhattan's wealthiest citizens, and the key to cracking the case is a mysterious sports car that is rigged like one seen in James Bond movies. Also, Mac receives phone calls from an unidentified caller who may be targeting him.
A groom is murdered on his wedding day and his body is wrapped like a present; and Stella finds a box of bloody puzzle pieces on her car, a discovery that causes Mac to fly to Chicago to solve the mystery.
It's no laughing matter when a murder victim is discovered with traces of disappearing ink and an exploding cigar. The investigation leads the CSIs to a notorious prank seller known as Laughing Larry. Meanwhile, Danny probes a case that involves a boy from his neighborhood.
Mac investigates the murder of a woman outside of a flower shop and then probes the death of man discovered two buildings down from the first victim. The CSIs work to determine if the two killings are related.
A fire breaks out in Stella's apartment building and she rescues a neighbor's son trapped in the blaze. Soon, it is determined that arson was the cause of the fire and that a charred body found in the building was likely placed there before the inferno began. The investigation reveals a convicted arsonist lives down the hall and that Stella's neighbor may be involved in criminal activity.
A prep-school guidance counselor who is serving as a chaperone for a dance is brutally beaten and murdered in a school lab. The investigation reveals the victim may have been involved with drugs and gambling. Meanwhile, Mac is pressured to find the so-called taxicab killer before he strikes again.
The CSIs search for the so-called cabbie killer after another one of his victims is discovered floating in a fountain. In another case, a hoops fan randomly selected from a drawing at an arena dies after sinking a basketball shot worth $1 million. The investigation reveals the victim was poisoned. Bill Raftery and Verne Lundquist appear as themselves.
In the fourth-season finale, a bank robbery goes wrong and a manager is gunned down. The botched attempt and shooting prompt a nervous gunman to take hostages and to demand that a CSI be brought in to prove he is innocent.