Shortly before Horatio is set to testify at a high-profile murder case in which the defendant is the husband of a city councilwoman, he's given new facts that could prove the man is innocent. The information revolves around the discovery of a second victim who matches the profile of the first one and was killed in a similar manner. If the man on trial isn't guilty, then Horatio fears that a serial killer may be on the loose.

Horatio battles the FBI and U.S. Marshals as he tries to uncover why a woman was murdered at the side of a pool at a hotel. The investigation begins shortly after the woman is found dead from two shots to the head. The evidence suggests that it was a professional hit man, which is strange enough, but the case becomes additionally puzzling when the Feds and U.S. Marshals get involved. Plus, the murder weapon is linked to a cold case that was orinially investigated by Yelina. Meanwhile, Calleigh talks to her dad about his drinking problem after she's called away from work to drag him out of a bar.

An explosion in a rundown barn nearly kills Horatio and his crew just after they find a dead man inside, bound with duct tape. The case gets personal for Horatio when he discovers that the leading suspect knew his late brother. Meanwhile, Alexx is shocked to learn that her best friend's husband has been murdered, apparently the victim of a random shooting.

An inmate at a massive Miami prison complex is stabbed to death in the prison yard. Shortly after Horatio arrives on the scene, a helicopter appears above the correctional facility. He immediately fears that the murder was a decoy to set up an escape. That's confirmed as three prisoners hop aboard the chopper and flee. As the investigation gets underway, the identities of the fugitives become known and a horrible realization sweeps over Horatio: two of the escapees are cold-blooded killers who already have their next victims picked out, and the other is a child molester who he recently put away.

The body of a man involved in recovering sunken treasure is discovered pinned to the wall of his new cabin cruiser by a bloody spear fired from a speargun at close range.

Horatio hunts for the person responsible for a vicious attack on a woman who was lured to a vacant condo and beaten with a piece of lumber.

The two young owners of a city bar are gunned down after hours and another man is wounded in the attack, which was allegedly committed by a masked gunman. Meanwhile, Calleigh's dad announces he is now a public defender and needs her help with a murder case in which a woman allegedly stabbed her boyfriend to death in his car with a screwdriver.

Horatio is driven to find his brother's killer and protect the niece he never knew existed when a woman shows up with Ray's illegitimate child. The team explores a voyeuristic Internet site in connection with a stockbroker's murder.

Someone uses a laser light to blind a pilot and bring down a jet, which carries $1.2 billion in checks to be delivered to banks on the eastern seaboard. The crime scene evidence reveals a victim, a young fundraising socialite, who was inside the plane but killed prior to the crash. Now, although the team has little to work with since the victim's wealthy family is opposing an autopsy, Horatio must follow the money trail. Meanwhile, Calleigh works on the case with her ex-boyfriend, Detective Hagen, and their romantic history could cause problems for both of them.

A former high school football star is murdered at a reunion, the CSIs must find the killer. The victim was the big man on campus in high school and there are a lot of guests at the reunion with reasons to resent him all these years. Now, the team must investigate everyone, from the head cheerleader to the school misfit, to see who held a decade-old grudge big enough to kill. Meanwhile, Horatio and Delko arrive at a crime scene and discover that it's already been processed, leading Horatio to uncover further information about his brother possibly being alive.

While investigating the murder of man hired to make dirty bombs that terrorists intend to set off in Miami, Horatio finds the bloody fingerprint of his brother, Raymond, who he believed to be dead. However, when Raymond's son is kidnapped, he emerges from deep undercover and the estranged brothers must work together to save the boy. Meanwhile, while processing a crime scene, an unknown assailant holds a gun to Calleigh's head and, after a deadly confrontation in her gun lab, she is left doubting that she can continue as a CSI.