At an apartment complex, the Superintendent, Mr. Evans, takes a couple to view an apartment, and tells them that he hasn't had time to clean it yet; he lets them in and they're shocked by what they see: blood all over the walls. The woman says to call 9-1-1... Brass tells Grissom that Clifford Renteria and his girlfriend, Alison Scott, are listed on the lease, but they snuck out in the middle of the night. Grissom points out that it could be animal blood; Brass doesn't buy it. Grissom tests it, and it comes back as human blood; he notes that a human has only 8 pints of blood, so whoever the blood belongs to, is dead.
Grissom examines the blood drops on the wall; Sara asks if there's any uniformity, and notes that from the shape of the drops, it looks like a high-velocity impact, like from a gunshot wound, but there's no bullet holes in the walls. Warrick sprays the floor with luminol; it shows that the floor was covered in blood. There are voids in the blood where the furniture would have been; Grissom notes what would have been the couch, a square that would have been the TV or a magazine, a triangle that might have been a lamp and Warrick finds an odd shape that he thinks looks like an electric saw.
At a forest fire, helicopters drop water on the trees; Nick and Catherine meet Det. O'Riley who points out a dead scuba diver up in a tree. They wonder how he got up there; Nick points out that Lake Mead is just over the ridge and the helicopters are dropping water... Catherine says that that's just an urban legend. Nick tells her that after today, he'll even believe in Santa Claus.
Brass tells Grissom that when they were at the apartment, they didn't know the full story; Mr. Evans, the Super, tells them that Alison Scott was gone before Clifford Renteria moved out, and adds that Renteria didn't leave a forwarding address. He tells them that when they find Renteria, they should tell him that he's not getting his security deposit back.
Catherine takes photos at the scene; Nick finds a void in the fire. Catherine comments that there was a consistent burn over the area, and the cause of the fire was a downed power line. Nick finds a match book with a cigarette butt stuck in it, and sprays it with hairspray to stop it from deteriorating.
At the Monaco Receiving dock, Brass and Grissom talk to Clifford Renteria, and ask about Alison Scott; he tells them that she left to visit her parents in Canada. Grissom mentions the disturbing evidence of the blood that they found in the apartment; Cliff says that it was all the Super's fault, and it serves him right, because the apartment smelled bad, there were flies all over, and the hot water didn't work, and the Super never did anything about it. He tells them that he has Hep-C, and gets nose bleeds. Grissom asks if he expirated blood all over the walls just to get back at the Super, and Cliff says yes, he did. Brass and Grissom go off and discuss whether they believe him or not; Grissom says that the blood drops are the wrong shape for what Cliff is telling them. They go back and ask for a blood sample from him, and tell him that they want to visit his new apartment. Cliff tells them that his furniture is in a rental truck parked at the loading dock, and he's been sleeping in that. They look over the furniture; Grissom notes an electric saw and a lamp with a triangle base; he asks if they can take it back to the lab, and Cliff agrees to let them.
At the lab, Sara comments that there's no blood observable on the tools, and Warrick comments that he probably ditched the tools he used anyway. They dip the tools in a blood-like substance and test the blood drops that they make. Greg tells Grissom that the DNA tests from Cliff's sample all point to the blood on the walls being his; Brass comments that Grissom said that it couldn't have been Cliff's, and Grissom admits that he's been wrong before. They interview Cliff again; Grissom double-checks that Cliff did this to get back at the Super, Mr. Evans; Cliff says that he did, and he'd blow blood onto the walls whenever something went wrong, like the hot water going off, or the toilet would break, and he'd know that the Super wouldn't fix it. He offers to show them how he did it, and puts a piece of newspaper up against the wall, and blows his nose. In the lab, Sara compares the spatter pattern to the ones on the walls of the apartment, and finds that they are the same; Grissom points out that Cliff stood very close to the wall, and was almost sideways when he did it, which changed the shape of the drops. Greg comes in with news of the DNA results from the lamp base; the blood is not Cliff's and it's from a female. Sara points out that Cliff's girlfriend is still missing, and Grissom says that the two things might have nothing to do with each other, and Sara adds, "or everything."
Catherine and Nick talk to Dr. Robbins about the scuba diver; he tells them that the diver didn't die in the fire, as he has no smoke in his lungs, which means that he stopped breathing before the fire started. He also comments that the injuries that he has are inconsistent with a fall from a helicopter, so Nick's urban legend theory is out. He tells them that the cause of death was a heart attack caused by a blow to his chest interrupting his heart rhythm. Nick suggests that he could have been clipped by the bucket of the helicopter as it was scooping up water. Dr. Robbins also tells them that there's soap resin on the diver's skin, like he started to shower and didn't rinse off after soaping up.
Nick takes a sample of the soap; Greg comes in and comments that it's probably dish washing liquid, saying that the wetsuits aren't easy to put on, but a cap full of dish soap really helps. He asks Nick about the safety belt, which Nick says is still sealed, and then they look at the air gauge on the oxygen tank; it shows that the tank was full, but the face of the gauge is broken. Nick comments that only one of two things will do that; excess heat or excess air; he has an idea...
Sara and Grissom go back to the apartment; Sara comments that Brass has checked with Alison Scott's parents who said that she never arrived at their home, and that it's possible that Cliff did the nose blowing to cover up the blood from her murder. Grissom checks the blood on the walls and notes that there's only one layer, which means there is no evidence to support Sara's theory. He notices flies in the room; Sara counts 15 or so of them, and he says that there were four on his initial walk-through. He checks the ceiling and finds fresh fly regurgitation; he says that the scene has been sealed, and the blood on the walls is old, so where are the flies getting the fresh blood? He then answers his own question by saying that it could be a rodent; Sara says it could be Alison. They look around the apartment and find more flies in the kitchen, particularly around a vent on the wall.
Catherine asks why Greg hasn't finished with the DNA that she gave him and he tells her that he's been working with Nick on the case. She talks to Nick about using Greg and cutting her out of the case; he shows her the oxygen tank, which has a fissure in it. He says that there was an explosion, and he's trying to find the serial number on the tank to see if that will lead him to the identity of the diver through a credit card rental.
Catherine, Nick and Det. O'Riley go to a house and ask to speak to Jerry Walden, the man who answers says that he's Jerry. They ask if he's missing an oxygen tank for scuba diving and he tells them that he lent it to Bruce Skeller, his business partner, and that Bruce was an experienced diver. Catherine tells him that Bruce didn't die in the water. Jerry tells them that he and Bruce were in business together, and had been friends since childhood. Catherine asks why Bruce would borrow a tank if he were an experienced diver; Jerry comments that just because he was experienced didn't make him any less of a free-loader. Catherine notices that there's something missing in the room; he says that he has sent his coffee table out to be refinished; Catherine asks what kind of wood it's made of and he tells her that it's ash wood. Catherine examines the floor and sees splinters of wood and comments that she thinks the table needed more than just refinishing.
Grissom and Sara find fly egg casings in the vent cover. Grissom pulls out a beetle, and says that it's very important because it feeds on decomposing flesh. The Super comes up, and Grissom asks if he can get permission from the owner for them to break through the walls; the Super is unwilling to do this, wanting to know who will pay, and saying that the owner won't let them do that. Grissom seals up the beetle, saying that they have to keep it alive in order to get the DNA from it, as it is their only hope of getting into the walls. At the lab, Warrick asks why they're extracting the stomach contents of the beetle; Sara tells him that they have to prove that it was eating human flesh rather than some other flesh.
Catherine talks to Paul Newsome, the district engineer, who shows her the land that Jerry Walden and Bruce Skeller owned; he tells her that they bought the land at a low price and sold high; he adds that Jerry was all for selling the land, but that Bruce was an environmentalist and didn't want to sell it.
At the lab, Ronnie, the handwriting analyst, tells Catherine that if the land deed is a forgery, then it's an excellent one. He shows her the original copy of the land deed, and that the signatures disappear under lights of different wave lengths, but the only thing that this means is that they used different pens to sign. He notes that otherwise, it looks completely legal, and it has been notarized.
Greg tells Grissom that there's human DNA in the beetle's stomach contents; Grissom gets Sara and Warrick and they go back to the apartment complex with their sledgehammers. Brass tells them that they have the warrant; they start to knock down the walls, but find nothing in the first one. Later on, Brass comes in and tells them that he has someone that they should meet: Alison Scott. She tells them that the blood on the walls is definitely from Cliff, commenting that this is one of the reasons why she left him. She tells them that she went back to her old boyfriend in Chicago; and confirms that the blood from the triangular lamp base is hers, saying that she hated the lamp and used to stub her toe on it regularly. They ask her to give a blood sample; she agrees, and tells them that Cliff is a lot of things, but he's not a murderer. Sara tells Grissom that they have to think outside the box; Grissom thinks for a few seconds then they go to the Super's apartment, and ask to look at his walls.
Greg tells Catherine that he has a profile from the torch used in the fire, and got DNA from the cigarette butt. Catherine and Nick find ash wood splinters in the scuba diver's back; they compare them to the splinters taken from Jerry's apartment and find that they match. Warrick speculates that Bruce and Jerry got into an argument over the land, Jerry punched Bruce in the chest, causing the irregular heart rhythm that was his cause of death, and also causing him to fall backwards into the coffee table.
Grissom and Sara examine the apartment of Mr. Evans; he tells them that there was some water damage that required the wall to be replastered. Grissom asks to examine the vent and Brass comes in with the new warrant that includes the Super's apartment, commenting that Mr. Evans neglected to mention that his wife was missing. Grissom notes that the Super shares the vents with Renteria's apartment, and Mr. Evans comments that he and Cliff were always arguing over the temperature. They break through the wall, and Sara finds a blanket with blood on it and bugs, and comments that he's hiding a body. Brass asks if he didn't realize how much a dead body would smell, and had to move it around; Grissom tells him to tell them where the body is, or else they'll rip the place apart.
Det. O'Riley talks to Jerry in the interview room; Catherine comes in and says that they know that he killed Bruce at his apartment as the forest where the body was found is all telarosa and oak trees, but the splinters in the body were ash wood, the same as his coffee table. She says that he killed Bruce and then dressed him in the scuba suit, took him up to the area of the forest fire and started another fire, figuring that the body would be burned and it would just be assumed to be part of the original fire. Nick tells him that the oxygen tank blew up because of the heat and that is what landed the body in the tree.
At the apartment, Warrick, Sara and Grissom continue to break through the walls and find nothing. Warrick finds a hollow section in the wall; they break through it, but it's only the fold-away ironing board. Warrick comments that they might be in the wrong apartment again, but Grissom says that he doesn't think he's wrong this time. He goes outside; Sara comes out to see if he's okay, and he tells her that he's angry because Mr. Evans knows where the body is and won't tell them. She suggests that he take a walk, and then wipes some plaster dust off his face, and comments that he should get cleaned up. He goes back inside to the bathroom and turns on the hot water, but nothing comes out; the cold water is fine. He asks Brass when Cliff said he lost his hot water; Brass says that he didn't say any specific date, and Mr. Evans tells them that they've had some hot water problems and the plumber is coming on Saturday. Grissom asks to see the water tanks in the basement; they go down and Grissom notes that they have a filtration system and there's no interruption in the cold water line, but the Super has cut off the water from the hot water tank to the filtration system. Mr. Evans says that he had no choice, as it wasn't working. Grissom knocks on the tank and comments that they're hollow, but this one doesn't sound hollow; he opens it up and finds the body of a woman inside, covered in bugs. Mr. Evans comments, "She nagged me."
At the lab, Sara and Nick ask Grissom if he wants to join them for breakfast; he tells them that he's going to eat with his beetle, saying that it solved the case.