Fifteen years ago, Catherine helped put John Mathers on death row for the rape and murder of a coed at a nearby college. Her body was dumped in a trash bag, and two other girls found the same way were also linked to the suspect. As the episode opens, Mathers is about to be executed when he's granted a stay based on new DNA evidence. But while Catherine awaits the results of tests on hair samples found on the first victim, another girl at the same school is murdered... and discarded just like the first three - meaning the wrong man could be behind bars.

Warrick investigates the drive by murder of a young girl in his old neighborhood. The girl's father is a friend and mentor of Warrick's as well. Warrick struggles to control his emotions to the point where Grissom pulls him off of the case. Nick tackles the murder of a computer company employee. The only supects are the three other employees in the building.

Catherine takes the point in the investigation of an escalating series of peeping tom incidents. The crew is racing to stop the attacker before his crimes become more serious assaults. Meanwhile, Sara reopens the dormant case of her friend, a district attorney. The DA is facing surgery to remove a bullet she got in an attack that killed her husband three years prior.

Grissom and Catherine investigate a movie-goer who was stabbed to death; the team comes to a forensic impasse while processing a teenager who died from a gunshot wound.

A woman's body is discovered in a small press box at a high-school football field, apparently the victim of strangulation. The evidence leads investigators to two convicts, one of whom is still in prison. At the crime lab, an explosion destroys key evidence and leaves Greg and Sara injured.

Things get personal for the CSIs in the top-rated series' third-season finale, which explores the intersection of the investigators' work and their personal lives. Three masked bandits steal a safe-deposit box from a Las Vegas bank in a daring daytime heist, during which Det. Lockwood is killed. Lockwood's death provides Grissom and his crew with added incentive to find the murderous culprits. But the case really hits home for Catherine when the evidence leads to casino owner Sam Braun, who is like a father to her. Also, Grissom's hearing gets progressively worse and he seeks out Dr. Robbins for help.

Ad aprire la quarta stagione di investigazioni dell'unità CSI è un episodio "doppio": in questa prima parte, i cadaveri di due coniugi vengono rinvenuti in una stanza d'albergo. Quando però Grissom e i suoi apprendono che i due si erano imbattuti in un'altra coppia di sposini la sera prima in un bar, cominciano a sospettare che via sia in circolazione un pericoloso duo di serial killer. A complicare il caso saranno poi le ingerenze della stampa e il grave errore commesso da un membro della squadra.

E' uno spettacolo davvero macabro quello che si presenta agli occhi di Grissom e Warrick, accorsi sulla scena di un crimine: si tratta del cadavere mummificato di un'insegnante in pensione. Nel frattempo, Sara e Nick indagano su un caso di stupro che riguarda una giovane ragazza, e che sembra essere collegato in qualche modo proprio al ritrovamento dell'insegnante. Catherine infine, cerca di scoprire la provenienza di una pistola rinvenuta da un ragazzo.

Dopo Brian Mobley, che più volte si è rivelato una spina nel fianco per Grissom, ecco arrivare Rory Atwater, il nuovo sceriffo di Las Vegas. Anche lui darà filo da torcere alla squadra CSI in relazione ad una irregolarità che rischia di mettere la polizia in cattiva luce. Tutto nasce da un coltello trovato da Warrick nella macchina di un sospettato. Il problema è che il mandato copriva solo la scena del crimine, ma non l'auto. Ciò rende la prova inammissibile. Grissom e gli altri dovranno così ricominciare tutto da capo, e avranno solo 24 ore per trovare nuove prove prima che il presunto killer venga rilasciato.

A woman found murdered in her home. Her resemblance to Sara causes Grissom to take a personal interest in the case and forces him to examine the decisions he has made in his life.

Grissom, Catherine and Nick investigate the death of a runner during a competition. Meanwhile Sara and Warrick investigate the death of couple in a hotel room, initially thought to be a murder-suicide. Both cases take place during this 'cop' competition , with more than 20.000 attendants (all of them cops). This gives the investigations an interesting turn, as almost all the suspects and all dead people are cops.

A wild night in Las Vegas has the CSIs simultaneously investigating four fatalities. Greg Sanders' first day in the field has him assisting Grissom's investigation of a fatal nightclub shooting. Meanwhile, Catherine probes into the death of a stripper found dead in a hotel room, while Sara and Nick look into the strange case of an "alien" body found near Area 51.

The killer from The Execution of Catherine Willows returns with deadly results. The entire team is engaged in a race against time to stop this sadistic killer from succeeding again. Grissom is additionally distraught by the idea that this killer is smarter than he is.

One night, four cases. Grissom and Brass get a stolen hummer and a taco stand. Warrick gets a dead car show girl. Sara and Greg get a dead bodybuilder. Nick gets a dead boy found on a bench.