At a house in the suburbs, the front door opens and a girl comes out, shouting for help, and runs off across the street, where lights go on.
Grissom arrives at the scene, O'Reily meets him and says that the press is going to be all over the story, as there are four dead: a mother, father, and two sons. The two girls in the family are alive; a teen-aged girl and her younger sister. O'Reily says that they couldn't have done it. An officer comes out of the house and throws up; Grissom asks what's wrong with the officers, and O'Reily says, "They've been inside."
Grissom and an assistant go in for a preliminary walk-through; Grissom wants to look, and have the assistant take notes. Grissom notes a heavy copper smell in the air, which means that there's a lot of blood, and tells the assistant to breathe through his mouth. Upstairs, they find a body in the hallway; the father, who was killed in the hallway and has multiple stab wounds to the back and neck. The assistant begins to look sick, and starts swaying as he's standing there; he tells Grissom that he has to go out to get some air. Sara shows up and takes notes for Grissom; she heard on the police scanner that there'd been a quadruple homicide. There's a blood swirl on the wall; Sara suggests a cult. They find the mother in the bedroom, still in bed, and Grissom determines that she was killed while asleep as she has no defensive wounds. In the boys' bedroom, they find one son dead in bed; Grissom checks the window, which is locked. The second bed is empty, but in examining another blood swirl on the mirror, Grissom sees the second body, between the bed and the wall, with two bloody hand prints on the wall above it.
Grissom tells the police that he wants the paramedics who arrived at the scene back at the house; he also wants his entire team to assist in processing the scene. The media is present, making reports outside the house, and neighbors are gathered around. Det. O'Reily talks to Tina, the teen-aged daughter, and wants her clothes for examination. She wants to be with her younger sister, saying that Brenda is scared; Brenda, sitting in a cruiser, is impassive to everything that's going on. Grissom tries to talk to Brenda, but she won't answer his questions. As he gets up to leave, she says, "the Buffalo," but won't say anything further when he asks who the Buffalo is. Grissom tells the team that there are to be no interviews and they're not to talk to the Sheriff either. He gives them their assignments: Catherine is to map the scene; Nick and Warrick are to do the perimeter.
In the house, Grissom checks the kitchen, the only room in the house with no blood in it. He and O'Reily find that there's a knife missing from the drawer; Grissom speculates that the killer had been in the house before, and knew where he was going. Grissom checks the floor and finds shoe prints and lifts some prints; he checks the prints against the shoes of the paramedics, and finds that they match. They admit being in the kitchen, saying that they went in via the front door, checked the bodies for a pulse, and then left. O'Reily tells Grissom that the news wants to talk to him; he says that he's not ruling anything out for now; the Sheriff wants to know what he has, and is upset when Grissom says that he has nothing, as the Sheriff wants to have something to tell the press; he compares Grissom to Ecklie, saying that Grissom should be more like Ecklie. Grissom tells Sara to transport Brenda to the hospital; she can't believe that in the biggest case of the year, she's just a taxi service, but Grissom tells her that he wants one of his team with Brenda.
In daylight, the team processes the scene; Warrick checks the garden, Nick photographs the doors, Catherine gathers blood. Nick finds a cigarette butt and burnt matches in the flowers, and Warrick finds tire tracks from a bike in the dirt. As the bodies are taken out, Grissom tells the morgue attendants that he wants all their personal effects. Grissom asks Catherine's opinion on the cult angle; she thinks it's an imitation, not the real thing. Grissom speculates that the mother was killed first, in bed; then the father was killed as he ran out of the bedroom to protect the kids, attacked first in the bedroom, and killed in the hallway. They both agree that whoever was left at the scene is their first suspect, and that's Tina.
Sara complains to Brass that she shouldn't be the one to be looking after Brenda, saying that Catherine's a mother, she should be with the little girl. Brass tells her to take Brenda to Sunrise for a psych evaluation. Brass and Grissom interview Tina to find out what she knows; she tells them that she heard footsteps and hid in the closet. Grissom asks if she has a boyfriend; she says that she's dated a couple of guys, but no one special, and none of them are called "Buffalo." Brass comments that in such a large family, she must be used to hearing footsteps at night and wonders why last night was different; she explains that she knows her family's footsteps and the ones last night were different.
Warrick takes prints of the tire tracks in the garden, and says that they're fresh, which he knows because of the timing of the sprinklers in conjunction with the time of the murders. He and Nick agree that it's bigger than a bicycle but smaller than a motorcycle, and Nick says that it probably goes with the butt that he found.
At the hospital, the doctor brings Brenda out and tells Sara that the psychiatrist should be there within an hour, but since the Child Services representative is there, Sara can leave. While they're talking, Brenda comes over to Sara and holds her hand, so Sara tells her that she won't leave. At the house, Catherine is mapping the scene, and is so involved in what she's doing that she doesn't realize what time it is until she hears the clock chime, and then remembers that she's supposed to pick up Lindsey.
Warrick tells Grissom that the tire treads belong to a '93 scooter, and a kid who lives three blocks away from the crime scene owns the same scooter. Grissom is concerned that there's no blood at all on Tina's clothes, and says that she must have had help if she did it.
Catherine arrives at home and finds Lindsey there with Eddie, who picked her up after her ballet class that Catherine was late for. She apologizes, explaining that she's been really busy on the quadruple homicide; as they talk, her beeper goes off, and she has to go back to work.
Ecklie tells Grissom that the Sheriff wants results, which is something that his team can produce, and Grissom's can't. Warrick interviews the kid who owns the scooter; he says that he doesn't know anything about Tina's parents getting killed, and he doesn't know where the scooter is as he shares it with three other guys; he provides the names for Brass.
While they eat lunch, the team discusses the case; Homicide is getting the three guys who co-own the scooter; Nick comments that no one in the family smoked, so the butt that he found might belong to one of the scooter guys. Sara adds that the psychiatrist said that Brenda is in a catatonic state, but during the exam she freaked out in response to the name "Buffalo."
Catherine questions Tina about the lack of blood on her clothes, noting that Tina had said that she'd hugged her mother's body, and tripped over her father's body on the way out. Tina can't explain the lack of blood, but insists that she's telling the truth. Grissom says that the evidence tells a different story. Nick and Brass interview the guys who own the scooter, one of them, Jesse Overton, admits having had sex with Tina, and while they talk to him, he pulls out a cigarette, which matches the butt that Nick found at the scene. Nick tells him that there's no smoking in the interview rooms, and takes the cigarettes and then asks for the matches too. They get a warrant to search Jesse's house, and find bloody clothes in the garbage. Nick fits the match that he found at the scene to the book of matches, and tells Warrick that he broke the case.
They question Jesse Overton using a lie detector, and he admits that Tina approached him, asking him to kill her family, and that he killed them. He says that Tina let him in and let him choose which knife to use. When asked if the reason for committing the murders, he agrees that it was because Tina's parents wouldn't let them see each other. The test shows that he was telling the truth about everything, except the reason why he did it. Nick says that it doesn't matter why they did it, and he doesn't care. Grissom says he cares; he doesn't like holes and wants to know what Jesse and Tina are hiding.
Grissom finds out that Ecklie took the personal effects and didn't pass them on; he tells Ecklie that he's way off base in taking the effects; Ecklie points out that Grissom kept the Sheriff out of the loop and that's a career killer. Grissom goes over the items, and finds a necklace with a buffalo on it. He phones Sara at the hospital to find out if there were any signs of sexual assault on Brenda, and has Sara take ultra violet photographs of her.
Catherine reviews the mapping of the crime, thinking out loud that the killer started with the mother, then cut the father in the bedroom and followed him to the hallway where he killed him as he went to protect Brenda. She notices something in the blood drops and takes a closer look. As she's about to call Grissom with the news that she's discovered, a Family Services worker comes in and tells her that Eddie reported her for regularly neglecting Lindsey for her job, and there's going to be an investigation. Catherine calls Grissom and tells him that there's a problem with the blood drops; they show that the father wasn't running towards Brenda's room, he was coming out of the room when he was killed. This means that the father was killed first, and the blood drops in the hall came from the knife as the killer went to the bedroom to kill the mother.
Sara shows the pictures of Brenda to Tina; they show extensive bruising. The tell her that they know that something bad was going on in the house long before the murders, and that they know that her father was in Brenda's room the night of the murders. Grissom asks what happened, and why kill her mother and brothers as well? She says that they should have protected her from her father, and that he sexually assaulted her when she was young, and she learned to deal with it, but she wasn't going to let him do that to ~her~ daughter. Grissom is shocked that Brenda is Tina's daughter, he asks who the father is. Tina just says that she was only 13, and no one noticed that her clothes were getting bigger; Brenda might hate her for what she did, but she did it to protect Brenda.
Sara and Brenda sit in the hospital; Sara takes her hand and Brenda smiles a bit. Catherine leaves, Grissom tells Nick that things will be normal in 24 hours. Grissom takes an apple and fills in a clue on Ecklie's crossword puzzle and then sits down to finish it. Warrick sees him and laughs.