When a K9 police officer named Mike Lynch has his throat ripped out on Halloween, all signs point to his partner, a German Shepherd named Sam, as the killer. Nick manages to calm Sam down and extract some DNA from his fur and jaws, along with a chunk of flesh wedged in the dog's teeth. Initially, it looks like the blood belongs to Lynch, but the DNA tests show that it belongs to an unknown male. Further evidence leads the CSIs to canvas the area around the crime scene for a hooker that called Lynch prior to his murder, and Sam helps them pick out Carly Green from the lineup. Carly was an informant for Lynch, but when her questioning goes nowhere, the CSIs let her go. She turns up dead in a hotel room, with other DNA evidence left at the scene. Nick again enlists the help of Sam's expert nose, and the dog eventually leads them to their killer, Vinnie Barso. When the case closes and Nick learns that Sam is up for retirement soon, he considers adopting the dog.