A UFO cult commits mass suicide in an underground complex shortly after apparently dumping a body in a garden. When the CSI team discover their bodies, they suspect two members are still alive and go looking for them.
CSI Nick Stokes investigates a murder of a family in a house outside Las Vegas city. In the house there is evidence of violence, blood and gun firing. The rest of the team thinks that the entire family was murdered, but Nick has the feeling that the little girl, Cassie, is still alive. So they start investigating and discovering the motive why the family was murdered (an illegal green house full of pot) and a chew gum in every place they go for searching clues. So Nick believes that Cassie left them a trace of gum, gum drops.
A woman is found dead on the floor of her house near her baby's cradle. The gun in her hand, as well as a suicide note and a fridge-full of breast milk, suggest that it was a suicide. The lack of blood on her hand, however, indicates that she was murdered. During autopsy, it is revealed that her hymen is still intact; she is still a virgin despite having given birth to a son. Meanwhile, Grissom is called in by Ecklie to refute an experiment involving insects which were used to create a time-line for a murder case.
The CSI team is called to help when Karen Mathews reports her son, Jesse, missing from a playground where he was playing on the swings. Witnesses report seeing a man with a blue baseball cap near the swings. Clues in the park bathroom include blood and the boy's underpants. The boy is eventually found in a store, accompanied by a woman with a man fitting the earlier description. But the couple with the boy claim that he is their son Adam, while Karen insists he is her missing son Jesse.
The CSI's investigate the death of a very hairy man. At a quick glance, the body can be easily mistaken for a werewolf. The man died from a gun shot wound. He, along with his sister, has a rare genetic disorder which causes excessive hair growth.
A man walks into a shabby apartment and shoots the occupant. On his way home, a teenager's car crashes into his car. Ally Sullivan insists on calling the police but the man refuses. As he leaves, he sees her texting her phone. Thinking that she's calling the police, he kills her. The CSIs have a hard time finding this killer as he knows how to cover his tracks really well. He is truly a professional.
A half-naked woman is found half buried in the desert with a branding on her arm. She has no hair and is missing her right hand. The woman turns out to be Lady Heather's estranged daughter. The autopsy reveals that the woman was a victim of an experiment and starved to death. The team investigates the Betz Clinic where the deceased underwent a sleep therapy session for her insomnia.
Excessive smoke coming from a fire place alerts the fire department, which in turns calls the CSIs as a charred body is found inside the chimney. The body is burnt beyond recognition. A gym membership card found on the body identifies him as the son of the house's owner. The opportunity to search the house leads Catherine and Warrick to look for any evidence about a one-year-old disappearance of a young girl who was on her way to this house before she vanished without a trace.
The team is followed by a reality TV crew as it investigates the vicious rape of a real estate agent. Thanks to a video surveillance system, suspicion falls on someone taped entering the building and leaving flowers at the victims door, but forensics clears him. Suspicion then falls on the possibility that the perpetrator is a fireman when they find trace evidence of a fireman's uniform.
A teenage boy, Marlon, is being prosecuted for a murder of one of his female classmates, Stacey. During his trial, his 12-year-old sister Hannah takes the stand, and admits that she herself murdered Stacey, not Marlon. The "CSI" team is now faced with a strange dilemma - two confessions for the one murder. Collusion seems unlikely. As Hannah is a genius, in high school already, even though her confession sounds convincing, some CSI members suspect that she is making up her confession and could not have physically committed the murder. It now has to be decided who ...
A criminal defense lawyer is murdered at her son's wedding and tied to the back of a car with beer cans. It is later revealed that she had a bad attitude at the bride, the groom's father, and basically everyone else.
The team investigates the death of a man with a designer suit and the keys to a Ferrari, found dumped in a back alley. Their search leads them to believe he went down fighting and was dragged through a hall with wet paint on the walls. Eventually, they are led to a service in Las Vegas that orchestrates the 'perfect weekend' for an unsuspecting target by uncovering their fantasies and hiring actors to start and play a part in those fantasies.
Mrs. Cutler is found dead, shot through the mouth, in her own home, which shows no signs of forced entry or disturbances. Her husband, Mr. William Cutler, is missing, only to be found later, alive, at his office where all of his co-workers were shot dead. He left the bloody scene with only a minor injury. All of this quickly becomes a hostage situation when Mr. Cutler, after shooting a security at a casino, holds a woman hostage.
While Jim Brass is in surgery the team has two cases to tackle. The first involves a man, found decapitated on train tracks, with a 19-inch waist. Further investigation reveals that he wore a wasp corset, as was fashionable with gentlemen during the Civil War. The other crime scene is in a hotel room where a man is found in his boxers, dead, on the bed. Scattered around the room are evidence of a wild night of alcohol, drugs, and sex. Ellie, Brass's estranged daughter, comes to visit her father from LA.