The episode begins on a street where people are walking around in their Halloween costumes. Two girls arrive and there are searching for the address where the party is supposed to take place. But then they are scared when they see a man with a gun. Two guys follow them and asks them if they want to party with them. They get out of that place. They then come to an empty street and they see a dog chewing on a body and it has blood on its mouth. They scream. The team arrives at the scene and the dog is continuously barking. Nick thinks that the dog went for the victim’s throat. They find a gun and a wallet on the victim; he is a cop and his gun hasn’t been fire.
He is Mike Nelson from the K-9 unit. Now, is that his dog? So much for a man’s best friend! They also find meth on the dead cop. Brass doesn’t think the dog did it. At the CSI office, Sergeant Varanski arrives to meet Russell. He tells Russell that Nelson and Sam, the dog, were partners for eight years and a great team. Nelson was off duty last night and the dogs are trained to attack suspects or defend themselves. Nelson’s truck is found and Greg collects evidence from it. He finds a piece of the dog’s nail and dried blood on the window. Looks like the dog clawed his way out. Nick wonders why take the dog along to buy drugs? Maybe for protection! Nick finds a Department issued envelope and finds a lot of cash in it. Next, Finn and Morgan are at another crime scene, where Barry Sloan is found murdered in his plush house.
He is a divorce lawyer. The 911 call was made, saying that shots were fired. When the team arrived, the shower was running but Sloan appears to be bone dry!! A shot to the chest and the shooter missed a shot as there is a bullet hole on the wall and based on the trajectory, the shot came from the other side of the window. Finn talks to Miss Young who called 911. She was doing her yoga and she heard two gun shots at 6.05. She at once knew it had to be related to Barry as a lot of people hated him. Once there was a hate note stuck to her window, but that was actually for Barry. But they got the wrong house. Then, Marjorie Randal arrives to talk to Finn; she has been Barry’s neighbor for 13 years. At the office, Nick is flustered as he is unable to collect evidence and he feels they need to open the cage.
Russell asks him to wit or Varanski d he leaves however Nick decides otherwise. He lets Sam out of the cage d manages to put a guard on Sam’s mouth he sees that something is stuck in between his teeth. Russell is in the morgue and Robbins arrives. He points out that there are a lot of defensive wounds. David feels that the dog must have knocked Nelson over, breaking his arm and rendering it useless. That is why nelson didn’t shoot the dog. At the other crime scene, Morgan finds the murder weapon and Finn marks some evidences. Finn tells Morgan that Barry was with a woman as there is fresh semen found on the sheets. At the office, Sam’s and Nelson’s tox reports come back clean. The money wasn’t stolen from evidence, but from his personal account.
Just then Henry arrives and tells them that the dog is innocent; the saliva found on nelson’s neck is human! So why was Nelson in crack town anyway? Nick learns from Varanski that Sam understands only French. Next, Morgan tells Finn that the print on the window matches with a Brent Walsh who is a casino owner and in middle of a divorce. Barry was his lawyer. Walsh says that Barry was sleeping with his wife. He got suspicious when Barry was handing over his assets to his wife, instead of protecting them. He then went to Barry’s place and saw him with his wife. He did not kill Barry. He only wanted him disbarred. Meanwhile, Greg tells Russell that a few hours before his death, Nelson got a call from a pay phone and the surveillance camera shows that it is a “working” woman.
Russell notices a yellow bandanna in her pocket; just like the one they found on Nelson. But the DNA proved that a man killed Nelson. Russell says that he could arrange the hookers from crack town to line up and Sam can sniff her out. Sam identifies that the bandanna belonged to Hooker no.4, Carly Green. They bring her in. they know that Nelson arrested her and she went to prison for nine months and so she had motive. But she says that nelson was the only one who cared about her. In fact she used to give him information about the dealers; she was his CI. Hodges tells Morgan that they should talk about the kiss but she doesn’t want to. Finn senses something is on and Morgan tells her what happened. Finn then tells her that the gun belonged to Barry and Walsh’s alibi checked out.
This brings Walsh’s wife, Sabrina Walsh under suspicion. In the morgue, Robbins tells Russell that Carly’s body was just brought in; she overdosed and was found in a motel. Nick and Greg go through the evidence collected from the motel. They find a bloody bandage. But there is an LVPD envelope, just like the one they found in Nelson’s car. But Varanski told Nick that Nelson didn’t work in narcotics for over a year. Robbins tells Russell that Carly didn’t commit suicide as the needle didn’t hit her vein; she was a regular user and she knew how to find a vein. There is a needle mark on her back which proves that she was murdered. Also, she was ten weeks pregnant. Morgan talks to Sabrina and tells her that a third DNA was found on Barry’s penis.
Sabrina tells her that she saw ladies panties on his bed and they got into a big fight; Barry was sleeping with one of his neighbors. She was angry and left, but she didn’t kill him. Russell tells Nick that the baby’s DNA matched with the flesh that Nick pulled out from Sam’s teeth. So, Carly was sleeping with Nelson’s killer. The bloody bandage also has the same DNA. It appears that Sam bit the killer. So he will need medical attention. Nick feels that Sam can identify the guy. Next, Hodges tells Finn and Morgan that he lipstick found on Barry’s shirt was forty years old. Finn brings in Marjorie for questioning. Marjorie admits that she killed Barry.
She says that after her husband left her, she fell in love with Barry who was years younger to her. After she heard him arguing with Sabrina, she went to meet him. she tried kissing him and he shove her away saying that she was old enough to be his mother. she felt insulted and she shot him. Barry had given her his gun as there were some break-ins in the neighborhood. Next, Nick takes Sam to the medical center in the nearby alley. Sam follows the scent and tracks down the killer standing in a queue for burritos.
The killer sees Sam and runs. Nick and the cops chase him. The killer then tries shooting at Nick and Sam leaps at the killer. The guy also has bite marks as Sam had bitten him while he killed Nelson. Nick is happy but he is shocked to see that Sam took the bullet while he tried to save Nick. At the station, Vinny says that he wanted to get rid of the baby and Carly went to Nelson for help. Vinny saw them together and since he was high on meth, he killed Nelson in front of her. And then he killed her too as he didn’t want to take chances.
Nick is at the hospital and Varanski tells him that Sam is a fighter and he made it through the surgery. Varanski also tells Nick that Sam is going to retire at the end of the year and so he might need a new home. Nick is more than happy to keep Sam with him. But there are few rules and he lays them down to Sam licks him in agreement; so, we have a deal. The episode ends.