On Halloween night, Danny and Lindsay head out to Amityville, where the Duncan family has been found slaughtered in their house. Sheriff Benson thinks the house is cursed, since a family died there in a similar fashion years ago. Benson postulates that Glen Duncan shot his wife Amy and their sons, Charlie and Tony, before committing suicide. While examining the scene in the house, Danny and Lindsay discover the Duncans' daughter, Rose, in the wall, a gunshot wound to her abdomen. The CSIs have her rushed to the hospital. Lindsay is spooked when an old woman accosts her in the house and tells her to get out. With the help of Dr. Hawkes, Danny and Lindsay determine that Glen didn't kill himself, meaning that someone murdered the entire family. Rose identifies the killer as a terrifying monster. Lindsay finds evidence of tree sap in a print lifted from where they determine the killer stood, and from there they are able to trace the killer to a tree in the front yard and into the attic. It seems apparent that the killer was hiding out there for a significant amount of time.
Lindsay runs DNA on a shirt she found in the attic and finds alleles in common with the old woman from the house who scratched her. Danny and Lindsay decide to look up the older murders in the house and discover that two members of the Willens family survived: mother Betty and son Henry. Henry, who has been in and out of jail for years, currently works for Amityville Animal Control. Lindsay is able to locate him by GPS tracking the collars of dogs he recently rounded up, and the CSIs find him and his mother at the Duncans' house, retrieving a suitcase full of bones. The bones belong to Henry Willens' sister, whom he accidentally shot as a child. Not wanting to lose her last remaining child, Betty hid the body, but when she heard the Duncans were intending to renovate the house, she sent Henry to retrieve the little girl's bones. Henry snuck in after the Duncans left to go trick-or-treating, but they came back early when one of the children got a stomachache. Henry panicked and shot the family.
Mac and Stella join Flack at the scene of a zombie flash mob, where one of the zombies has fallen down dead. Dr. Hammerback determines he was the victim of both blunt and sharp force trauma, and ultimately died of a subdural hematoma, which probably didn't kill him until about fifteen minutes after the fatal blow. Though the victim's body temperature indicates he died eight hours ago, witness testimony suggests he died just three hours ago. The CSIs are able to identify the victim as Dexter Nevins and learn a funeral was held for him the day before. Adam also notes that oddly enough, the victim was wearing a diaper. The CSIs exhume his coffin and find that he clawed his way out. Inside the coffin they discover a small voodoo doll with rare Haitian blue coffee beans inside it.
Mac and Stella visit a voodoo shop where Mac is thrown by the number 333 appearing on the eyelids of Josephine Delacroix, the proprietor. Josephine, whose shop sells the rare coffee beans, admits to making the doll for Dexter, and Mac begins to suspect the man was faking his death in an insurance scam. Sure enough, Hammerback determines he died the first time around of congenitive heart failure caused by a drug in his system called tetradetoxin. Mac makes note of the name of the doctor that signed Dexter's death certificate, Roger Burgess, and when the CSIs discover he lived close to the graveyard, they pay him a visit. They find both Roger Burgess and Sophia Nevins, Dexter's wife, locked in an embrace in the bed--barely alive. Dexter rose from the dead and caught them in the act together. He dosed both with tetradetoxin but Roger hit him several times with a cricket bat before collapsing, leaving Dexter to stumble out and join the zombie walk before his own fatal fall.