Following a massacre at Miskatonic Medical School in Arkham, Massachusetts, two surviving physicians, Dr. Herbert West and Dr. Dan Cain, volunteer their services in Peru during a civil war. Although enemy troops are approaching, Herbert West refuses to retreat, certain he is on the verge of a breakthrough in his efforts to re-animate the dead using the organs of reptiles. While their Italian colleague, Francesca, runs to a rescue helicopter, Herbert repels the invaders with a machine gun and drags the wounded Dan Cain to safety. The doctors return to Miskatonic Hospital, where Dan treats a cancer patient named Gloria. Although Dan hopes to cure Gloria, Herbert believes the terminally ill woman would be useful in his experiments. Meanwhile, Police Lieutenant Leslie Chapham presents pathologist Wilbur Graves with the severed head of massacre victim Dr. Carl Hill, which he found at a carnival sideshow. Dissatisfied with the official explanation for the massacre, Lt. Chapham insists on examining the victims’ remains, which are stored in a special vault. Dr. Graves notes that no decay has occurred over the ensuing months, and the cells remain viable. Also in the vault is a bottle of luminous green liquid developed by Dr. Hill, the nature of which Dr. Graves claims no knowledge. Lt. Chapham continues his investigation at the hospital crematorium, as Ernest, the technician, incinerates the body of a suicidal ballet dancer who amputated her own feet. The detective’s suspicions intensify when he learns the feet have disappeared. Although Dr. Graves dismisses the idea of theft, Ernest hints that it has become a common occurrence. When Herbert West enters the vault and discovers Dr. Hill’s head, he wonders what brought it back. Despite his disdain for Hill, whom he labels “a plagiarist and a fraud,” Herbert compliments the doctor for proving that every part of the body maintains a degree of consciousness. That evening, Dan Cain returns home to the former mortuary he shares with Herbert West. He enters the basement laboratory to find his roommate emerging from a stone wall, which the house shares with a crypt in the adjacent cemetery. Using an iguana, Herbert demonstrates a vaporous drug that simulates a fatal heart attack, then removes the lizard’s amniotic fluid, from which he creates his re-animation serum, based on Dr. Hill’s formula. Herbert proves its effectiveness by bringing to life a creature made of severed fingers and an eyeball. Dan is appalled until Herbert presents him with the preserved heart of Megan Halsey, Dan’s girl friend who was killed in the massacre. Herbert intends to transplant the heart to a new body, and recruits Dan to help steal body parts from the hospital, certain their project will be overlooked by police. Within seconds, Lt. Chapham appears at the door, questioning the doctors about missing body parts and rumors of re-animation experiments. They dismiss the detective’s concerns, claiming their studies are purely theoretical. At the hospital, Dr. Graves revives a dead bat using Hill’s serum, and when the animal attacks, he amputates its wings. He tests the serum on Dr. Hill’s head and it speaks, recalling Dr. Graves’s lecture on the origin of disease as “totally idiotic.” Hill then demands help with “unfinished business.” Meanwhile, Herbert and Dan steal the body of a young woman by transporting it through the hospital lobby in a wheelchair as if she were a live patient. When they reach the parking lot, Dan encounters Francesca, who is seeking work in nearby Boston, Massachusetts. After she and Dan make dinner plans, Lt. Chapham appears, feigning interest in Francesca’s dog, Angel. The detective questions Francesca on her relationship with the two doctors, then takes her to the hospital psychiatric ward, where he recounts the details of the massacre. Little remained of the victims except the head of Dr. Hill, which continued to twitch long after being severed. Lt. Chapham also notes that the three mental patients blamed for the killings had all been declared dead, including the detective’s late wife, Elizabeth. As they enter the ward, Elizabeth Chapham attacks Francesca, but she is pulled to safety by the lieutenant. That evening, Herbert and Dan transplant Megan Halsey’s heart to the stolen cadaver. Francesca arrives and prepares dinner, then makes love with Dan while Herbert hides one of his failed experiments in the neighboring crypt. Lt. Chapham enters the basement and accuses Herbert of tampering with Elizabeth’s body. Herbert reveals that the detective beat his wife to death, then kills him with the heart-attack vapor. Alerted by the commotion, Dan enters the basement as Herbert injects Lt. Chapham with the green serum. The detective goes on a violent rampage and aims his gun at the doctors. Herbert amputates Chapham’s hand, and the lieutenant runs from the laboratory, encountering Francesca and Angel on the staircase. Lt. Chapham removes one of the dog’s forelimbs while beating it to death, then retreats to the cemetery. Although Herbert re-animates Angel, Francesca is horrified to discover that the dog’s missing limb has been replaced with Chapham’s hand, and declares her hatred for the doctors. Later, when Gloria succumbs to her illness, Herbert orders Ernest to remove the body. Upon his return home, Herbert surprises Dan by completing his assemblage of body parts with Gloria’s head. At the hospital, Lt. Chapham warns Francesca of his plans to kill Herbert and Dan, then holds Dr. Graves at gunpoint, ordering him to attach the severed bat wings to Dr. Hill’s head. Meanwhile, the re-animated mental patients escape, and after leaving a trail of carnage in their wake, they stagger toward the cemetery. As Herbert and Dan inject their creation with the life-giving serum, a crate is delivered to their door, containing Dr. Hill’s flying head. Herbert inadvertently releases the head, and while searching the house for it, Gloria gradually comes to life. Francesca enters the house, warning Herbert of Chapham’s threat, and they retreat to the laboratory. Gloria becomes jealous of Francesca, who considers the artificial creature an affront to God. As Chapham, Hill, and the mental patients force their way into the laboratory, Gloria’s new body quickly rejects its disparate parts. In the ensuing melee, an overturned vat of green serum seeps into the neighboring crypt, and the laboratory is overrun by Herbert’s experimental creatures, which drag him through the stone wall. He is killed in the subsequent collapse, while Hill and his fellow undead are trapped in the rubble. As Francesca and Dan escape to the surface, Megan’s heart stops beating.