Catherine and Nick become absorbed in the dangerous world of street racing after one of the illegal sport's hotshot drivers is found dead out in the desert at an abandoned airstrip. Back in town, Grissom and his crew probe the sudden death of a renowned poker player who suffered a violent convulsion in the middle of a high-stakes game with three others. It looks like natural causes, at first, but Grissom suspects foul play.

Grissom and his crew are called in to investigate the death of a young woman found in the hotel room of popular actor Tom Haviland. Haviland admits that he slept with her, but denies killing her. Grissom believes otherwise, however, after finding blood evidence on the celebrity. Then things get personal when Haviland's lawyer hires Grissom's former mentor, Philip Gerard, to find mistakes in the forensic part of the case. And, unfortunately for Grissom, Gerard finds plenty of errors to exploit.

Grissom and Catherine investigate the murder of a wealthy couple who are discovered just outside of their glamorous Las Vegas house, which was up for sale. Meanwhile, Sara probes the death of a cheerleader who is found eviscerated on a soccer field, her organs having been ingested by her killer.

Grissom suspects murder, not suicide, when the body of a dwarf is found hanging from a rope high above a stage in a casino hotel where a convention of little people is being held. Across town, Catherine is attacked at a crime scene where a man was murdered during what appears to have been a home robbery.

Grissom investigates when a woman vanishes while taking part in a magician's act. The female volunteer is placed inside the Amazing Zephyr's "Chamber of Doom" and the illusionist thrusts swords through the box. When it's opened, she's gone---and doesn't reappear in her seat as she's supposed to. Only traces of blood are left inside the chamber. Clues lead the CSIs to an eerie, decrepit mansion filled with secrets. Elsewhere, a rock star's death on a band bus appears to be the result of an overdose, but Catherine uncovers evidence that points to murder.

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.

A middleweight boxing champ dies in the ring after taking a brutal beating from his nemesis, but Grissom finds evidence that points to murder. Outside, in a nearby parking lot, Catherine works a crime scene where a Los Angeles gang member was gunned down, apparently by a rival gang. Also, Nick investigates a jewelry-store robbery.

Catherine called in by a pornographic-film developer to view graphic footage from a snuff movie in which a woman is brutally slain. But the clues are scant: The murderer isn't clearly seen, the cheap-looking hotel room could be almost anywhere and the film was sent to the developer anonymously. Outside of town, Grissom investigates another killing when a body, smothered with fire ants, is found stuffed inside a large toolbox. The initial findings indicate that the person died well over a year earlier.

Distracted by an attractive passenger, an Indian taxi driver feels his cab strike something at a suburban intersection. When he gets out to check, he finds a teenage boy pinned beneath the left rear tire, apparently killed on impact. The cabbie jumps back into the taxi to call for help, but a mob of angry onlookers - all of them white - drags him out and beats him for the crime. When Grissom arrives on the scene, he discovers that the teen has been eviscerated and has stab wounds, but shows no signs of having been struck by the cab.

Grissom, Warrick, and Nick investigate the case of man who apparently was murdered and pushed off a six story building. Grissom's involvement is cut short by renewed hearing problems. Catherine looks into the murder of man outside of his regular bar.

Grissom and Catherine investigate the grisly meat grinder murder at a slaughterhouse. Warrick and Sara look into the apparent suicide of a troubled young woman.

Some bird watchers find a raven with a human eyeball in its beak. This eventually leads our CSIs to a divorced man and his 2 children but not much else. Warrick and Grissom look into the circumstances of David's unusual and first autopsy. This takes them to a car dealership where the man worked.

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 investigates the murder of two men that leads him to a new encounter with Lady Heather. Catherine is desperate to find out what lead to the near death of her daughter and why her ex, Eddie, disappeared.

A man driving with a wooden stake in his head dies in front of a casino. Grissom and Nick work to discover the 'vampire' killer. Warrick and Catherine hunt for the killer of a NBA star's 5 year old son.

An elderly woman crashes her car into a crowded restaurant. One of the patrons is Sara's paramedic boyfriend. Grissom and Nick investigate the possible carbon monoxide murder of a woman in her home.

Catherine, Nick, and Sara investigate a seemingly months old body found in the mountains. Discovery of the man's identity lead the team to the world of robot wars. Warrick and Grissom look into the case of a man found murdered in an alley. Complicating their efforts, the medical examiner ""lost"" the body for 11 days. Greg helps this case by doing some undercover work.

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.

Grissom and Catherine investigate the murder of a despised comic who died onstage, apparently after drinking from a tainted bottle of water. The case grows more puzzling when a 15-year-old boy dies in a convenience store after drinking the same brand of water. Brass reopens a woman's accidental-death case after spotting the deceased's husband in a flashy new car with a trophy date on his arm.

A murder aboard a luxurious 747 and the discovery of two formally attired teens in the desert confound Grissom and his crew in this absorbing episode. First, a horse trainer is found dead in the cargo bay of a private jet carrying 12 well-heeled passengers and a million-dollar show horse. Initially, it appears the victim was trampled to death, but Grissom and Catherine uncover evidence of foul play. Meanwhile, in the Hell's Gate section of Death Valley, Sara and Warrick probe the apparent suicide of a 15-year-old boy found under a bedspread. The investigation intensifies when a girl's body turns up a half-mile away.

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.

Nella continuazione dell'episodio precedente, alcuni indizi portano Grissom, Warrick e Nick a sospettare che dietro gli omicidi si celi la figura di un poliziotto. Contemporaneamente, Catherine e Sara cercano di scoprire quale mistero si nasconde dietro all'uccisione di uno studente il cui corpo viene ritrovato in stato di decomposizione dentro una vasca da bagno.

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.

Mentre Las Vegas è investita da una terribile ondata di caldo, Catherine si occupa del caso di un bimbo di 6 mesi morto a causa della temperatura elevata sviluppatasi nell'auto in cui era stato lasciato dal genitore. Nel frattempo Warrick indaga sulla morte, apparentemente causata da un attacco cardiaco, di un uomo nella sua abitazione. Nick e Sara devono scoprire cosa si nasconde dietro al decesso di una ragazza trovata nel Lago Mojave.

Grissom e Catherine accorrono sul luogo in cui, dopo un incidente frontale, un uomo viene trovato morto sul ciglio della strada con indosso un costume da procione. Gli indizi conducono quindi i due investigatori alla "Fur Con", convention annuale che raccoglie tutte le persone che amano travestirsi da animali. I principali sospetti ricadranno immediatamente, a causa di alcuni peli blu trovati sulla scena del crimine, su Miss Kitty, che indossa un costume da gatta dello stesso colore.

A Jackpot, piccola e sperduta cittadina del Nevada settentrionale, un cane trova una testa umana. Grazie a una serie di rilievi sulle tracce di materiale vegetale presenti sul cranio, Catherine riesce a individuare la zona esatta in cui la testa è stata nascosta, dopodichè Grissom si reca sul posto per indagare. Scoprirà però che la cittadina nasconde segreti ancora più intricati, e dovrà confrontarsi duramente con le autorità locali, decise a ostacolare la sua attività investigativa.

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.

Quando Julie Waters, giovane modella alle prime armi, sparisce improvvisamente, Captain Jim Brass e i suoi uomini fanno irruzione nell'appartamento del "sospettato numero uno": si tratta di Charles Delhomme, fotografo di moda che ha lavorato spesso con la Waters. Delhomme in un primo momento confessa il rapimento e l'uccisione della giovane, poi cambia idea. A complicare le cose sarà inoltre la sua predilezione per Catherine e la disponibilità a parlare solo con lei.

Quando un'autobomba uccide tre persone davanti a un casinò, lo sceriffo di Las Vegas Rory Atwater, vera spina nel fianco dell'unità CSI, sospetta immediatamente che si tratti di un feroce atto terroristico. Grissom è però di un altro avviso, e intende dimostrare che dietro la strage si nasconde un altro movente. Intanto, Sara e Warrick hanno il compito di scoprire chi ha ucciso la moglie di un noto cantante di Las Vegas.

A teenager is found murdered on a construction site. Grissom, Sarah, and Warrick follow the lead of blood on a hammer. Nick, with an assist from Catherine, tries to find how a woman was killed by a gunshot on her lawn when no one nearby had a gun.

The lone holdout on a jury is found dead in the jury room. The eleven jurors who were voting guilty and who hated the dead juror are the natural suspects. Nick reopens a four year old case when the sister of the victim comes forward with new evidence.

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.

A mutilated female body is discovered tied to the undercarriage of a bus carrying prisoners from a women's correctional facility after a severed arm flies out from beneath the vehicle and strikes a car. Catherine leads the investigation and at first believes the woman was escaping, but the evidence increasingly points to murder. Back in town, Warrick probes the death of a gambler who was stabbed multiple times in his brother's kitchen.