A young woman dies from carbon monoxide poisoning but the man with her survives, in what appears to be a suicide pact related to their credit card debt. Meanwhile, Delko starts seeing his dead colleague, Tim Speedle.
The mother of Horatio's child surfaces and becomes a prime suspect in her husband's murder. She also comes under suspicion when the witness in Horatio's son's trial suddenly disappears before she can testify.
The body of the witness Julia Winston paid off in Horatio's son's trial is found in a swamp, but Horatio's investigation is interrupted by his extradition to Brazil to face murder charges. Meanwhile, Calleigh is kidnapped.
Calleigh's kidnappers use her to try and cover up a shooting death that they committed. However, she discovers that the gunshot is not what killed the victim. Meanwhile, CSI uncovers more clues in the death of the trial witness.
A notorious private investigator becomes a central figure in an investigation into a phony 911 call that ends in death, and he uses information he's gathered about Calleigh and Eric to get them thrown off the case.
A man falls to his death from a skyscraper window, and the new M.E. is shot and killed at the scene. The crimes are soon connected to ammunition dealers, an ATF agent, and Horatio's son Kyle's new stepfather.