The movie opens with title and credits running over a long shot of a tropical Pacific Island village of huts.
The opening scene is of a palm covered beach and cuts to an assembly of natives performing a ceremony. The seated Witch Doctor, Tano (Robert Swan), stabs a voodoo doll in the heart with a knife. Tano pronounces sentence on Kimo (Gregg Palmer), "Kimo you have committed the greatest crime of all. You have betrayed your own people. You have caused the death of your own father, our mighty chief. For these sins you must die." Kimo is staked to the ground. He proclaims his innocence. His real crime is consorting with the American Scientists and favoring their modern medicine over Tano's primitive cures. Chief Maranka (Baynes Barron) concurs with Tano. Kimo blames Tano and Maranka in a conspiracy to kill the old chief and take over the tribe. Unfortunately Kimo relies on testimony of his unfaithful wife, Korey (Suzanne Ridgeway). She makes him look even more guilty, sealing his fate. Kimo proclaims, "Korey, why have you betrayed me? I will come back from the grave to revenge for myself. Tano, you can kill my body but my spirit will never die. Tano, your days are numbered." A stake is driven into his heart. Chickens peck around for food next to his dead body. A ritual dance is performed. It appears to be a hula dance. Mrs. Mae Kilgore (Linda Watkins) was walking by the village and witnessed the proceedings. This is a taboo and punishable by death. Four natives carry a coffin and place it next to Kimo.
At the American Field lab, Dr. William Arnold (Tod Andrews) and Prof. Clark (John McNamara) complain of the noise of the tribal drums, the malaria, jungle rot, fever, heat and the ignorance of the natives. Arnold explains he tried his best to save the chief, but he was too far gone to be saved. They grow concerned for their own safety. Professor Clark is a seasoned field researcher, but he would like to return to the states also. His ideal job would be as a chemist at a distillery. Needless to say he likes his distilled spirits. An atom bomb was dropped about 1,500 miles away, but because of a freak typhoon the fallout was carried to this little island. But it isn't fallout killing the natives but plague. Geiger counter readings indicate only slightly above normal background. Dr. Arnold wants to go home but his girlfriend wants to continue with her career. Eddie (Mark Sheeler) is a Sergeant. He enters the lab after fixing the camp generator. He helps himself to a cup of coffee. The three are enjoying their coffee when they hear Mrs. Kilgore scream. She is being chased by a native who tries to kill her. The three chase the native away and carry Mrs. Kilgore into the lab. Fortunately she was just frightened, not seriously injured.
The native ceremony continues. Kimo's body is placed in the wooden casket vertically. He still has the ceremonial dagger in his chest. Four men carry him to the cemetery and bury him. The village members follow the casket. Mrs. Kilgore wakes babbling, and then tells Dr. Arnold, Professor Clark, and Eddie they killed Kimo. Mrs. Kilgore resumes her composure and her old self--now she is concerned about her appearance. The twice married and widowed woman is on the prowl for another husband. At the graveyard, Tano throws the voodoo doll of Kimo, some bones, and dirt into the open grave. The natives fill the remainder with dirt. Dr. Arnold reminds Mrs. Kilgore that she passed the cemetery during a native ceremony and that is why she was attacked. She tells Dr. Arnold she was just coming to get some more "Nerve Medicine." She runs the local trading post and is a frequent guest at the American Compound.
A helicopter departs a Navy ship offshore. On board is Dr. Terry Mason (Tina Carver). They land and are met by Dr. Arnold and Eddie in a Jeep. They pass the cemetery on the way over and we see the ground over Kimo's grave begin to erupt slowly. Eddie loads Terry's bags and equipment into the Jeep. Arnold warns Terry that the natives are getting dangerous. They drive past the cemetery again and this time the crack in the ground has widened. They arrive back at the compound. Terry meets Mrs. Kilgore and the two strike-up an instant friendship. Mae takes Terry to her quarters to freshen up. She is introduced to an outcast native girl, Orchid (Grace Mathews). Mae asks Eddie to take her home. Terry enjoys a shower. Dr. Arnold comes to collect Terry, but she isn't quite dressed yet. Two natives arrive for medical treatment. Norgu (Lee Rhodes) accompanies his wife Dori (Lenmana Guerin) for another round of skin treatments. Her face is scarred. It is a side effect of the plague and mild radiation burning. Terry arrives at the lab and is introduced to Dori. Terry examines her and concurs that Dr. Arnold's treatment is working, but she may have something else that may work better. Arnold tells Dori and Norgu to return tomorrow for her first new treatment.
Korey, freshly widowed, goes to visit her lover, Tano. He rebuffs her advances now. She states, "It was for you that I helped kill Kimo, my husband. And since he died the fire of your love as grown cold." He coldly replies, "I could never have a traitor like you as my wife. Get out of here." His new interest, Naomi (Tani Marsh) returns from the jungle with poison berries to help Tano make poison darts. Tano and Naomi depart after Tano tells Korey he could just as easily kill her as the Americans. Korey departs in a jealous huff.
Terry and Arnold finish lunch and go for a walk. Arnold is trying to woo Terry into a marriage in which she isn't interested. Terry notices a stake and skull topple over in the cemetery. She wants to investigate. They notice a strange plant or tree stump growing out of the ground over Kimo's grave. Arnold suggests Prof. Clark look at it. They walk back. The next day they show Clark a drawing. Norgu and Dori arrive and Norgu is shocked when he sees the drawing. "Kimo's curse has come true. His spirit returns from the grave," a frightened Norgu mutters. He goes on to describe a tree monster, a Tabanga--a creature of revenge. Orchid comes into the lab. She reports the strange growth from Kimo's grave. She adds that it has a ceremonial dagger in its heart. Norgu accompanies Terry, Arnold and Clark back to the cemetery to examine the stump. It is now more exposed and seems to have a face. It is highly radioactive and has a heartbeat. Norgu tells them, "The Tabanga will soon remove itself from the ground." Terry observes, "You know what, I have an eerie feeling that this thing knows what were saying." They receive a radio message from the foundation headquarters in Washington asking them to remove and study the tree growth. This pleases Terry.
Tano and Chief Maranka decide that Norgu has disobeyed the taboo and must die with the Americans. Tano advises caution. Tano plans to poison the Tabanga into becoming his slave to kill at his command. The chief reminds Tano that Korey must also die. They don't realize Korey was eavesdropping outside the hut. Korey runs to the American compound. She tells the Americans, "Tano and Maranka are planning to kill all of you." Norgu questions her intentions and veracity. She finally admits she is on the death list also, and asks for sanctuary. Norgu vouches for her. Clark, Arnold and Terry return to the cemetery to remove the Tabanga. It is now seven feet tall and the roots are now fairly shallow and exposed. Its beating heart is visibly pulsating. They return it to the lab and place it on a gurney. The pulse weakens and Terry is convinced it is dying. She wants to try something radical. Dr. Arnold just wants to pack it in and throw it in the quicksand. Formula 447 is an experimental medicine used to restore heart action after radiation exposure. It works very slowly. They give the Tabanga an IV of the formula. The time is 10:00 p.m. They agree to check it in the morning after a good night's sleep. Just before 6:00 a.m. the lab is shown to be a shambles. Arnold, Clark and Terry arrive only to discover their "patient" is gone. The Tabanga (Chester Hayes) is out wandering the island, but the scientists don't know that yet. Clark postulates that the natives came in, took the Tabanga and trashed the lab. Eddie is the first to notice the radio room was also destroyed.
Naomi is out in the lake swimming. A very jealous Korey is out for a walk and spots her. She pulls her knife and approaches Naomi. Korey confronts her rival, "You've been hiding from me a long time. I've been waiting for this chance." They struggle and Naomi is getting the worst of it. Neither is aware they are being watched by the Tabanga. Korey strikes out at Naomi with her knife, but misses the mark, and drives the blade into a tree. Naomi retrieves a stick and knocks Korey out right into the waiting "arms" of the Tabanga. Naomi backs away in horror. Korey wakes, becomes aware of her predicament, and screams. She faints. Naomi runs back to the village to warn the others. The Tabanga carries Korey to the quicksand and tosses her in. He gets his revenge--one down and two to go. "I just saw the Tabanga," Naomi tells Chief Maranka. Tano and Naomi go to the cemetery to confirm her report. Sure enough the Tabanga is gone leaving only severed roots. Tano's plan is now to kill the Tabanga first, then the Americans. They return to the site of the girl fight. They find Korey's knife still in the tree and a piece of fabric from her dress.
Chief Maranka is sharpening a spear when the Tabanga approaches his hut. He throws his spear and misses from five feet away! The Tabanga presses him against a tree crushing the life from his body. Orchid comes to the lab from the village to report that the Tabanga is not dead. It killed Maranka. Terry is pleased and excited to hear her "patient" is alive. She dials back her enthusiasm when she hears it's on a killing spree.
The natives dig a pit and cover it with brush. They intend to trap and burn the Tabanga. Tano proclaims, "I am the bait that must lead him to the trap." He tells Maku (Chester Hayes in a dual role) to climb a tree and signal when the Tabanga approaches. Maku alerts Tano of the approach of the Tabanga. Tano leads it into the trap. The village men toss torches into the pit starting a fire. A short time later, a smoldering Tabanga emerges from the pit and continues his search for Tano. Maku and another native see it and run to the American compound for help. The Americans respond cautiously to the two men pounding on their door. Maku pleads, "Come quick, please help us. We cannot kill the Tabanga. We burn Tabanga with a mighty fire, but it didn't help. It came out alive, please come before it kills us all."
Tano is walking and spots the Tabanga. He backs away. Mrs. Kilgore arrives for the safety of the lab. Eddie, Clark and Arnold intend to help and the women join the men, all are well armed. The Tabanga chases Tano. He falls over a log and hits his head on a rock. He is stunned. This gives the Tabanga the chance it wants to seek revenge on the last of the three. It carries Tano to a cliff and throws him over. He is impaled on a stake and dies. Maku, another villager, and the Americans walk single file through the jungle. Mrs. Kilgore jabbers non-stop. Terry is bringing up the rear. Terry stops to rest and remove something from her shoe. Mrs. Kilgore continues to talk. She is unaware Terry is not with them. Terry gets her shoe back on and leans against the Tabanga. It grabs her and she screams and then faints. They run back as the Tabanga carries her away. She drops a shoe. They find her shoe, but Terry and the Tabanga are gone. She screams again and they follow the sound. She struggles. Eddie takes a couple of shots at the creature. He hits it but it has no effect. A few more shots just annoy it into dropping Terry and turning to face its tormentors. Dr. Arnold, Clark, and Eddie start firing. They are aiming for the ceremonial dagger. After a few dozen rounds, Dr. Arnold scores a direct hit on the knife's butt or pommel of the handle driving it into the heart. The Tabanga drops backwards into the quicksand. Terry runs into Dr. Arnold's arms. The Tabanga sinks into the quicksand. Maku thanks the Americans for their help and departs. Dr. Arnold and Terry kiss. We close with Mrs. Kilgore shamelessly trolling for husband number three. She asks Professor Clark, "By the way Professor. I never asked you. Are you married?"