Plot
On the Amazon River, a poacher hides from an unknown creature in his boat. When it breaks through the boat and corners him, he commits suicide with his gun.
A film crew is searching for the Shirishamas, a long-lost indigenous Amazonian tribe. The group consists of director Terri Flores, cameraman and childhood friend Danny Rich, production manager Denise Kalberg, Denise's boyfriend and sound engineer Gary Dixon, narrator Warren Westridge, anthropologist Professor Steven Cale, and boat skipper Mateo.
The group encounters stranded Paraguayan snake hunter Paul Serone, who claims he can help find the Shirishamas. Most of the crew are uncomfortable around Serone, and Cale clashes with him several times about Shirishama lore. Eventually, Cale is stung by a wasp, and an allergic reaction swells up his throat and leaves him unconscious. Serone performs an emergency cricothyrotomy, saving Cale's life. Soon after, with Gary's help, Serone takes over the boat, revealing his true goal: finding and capturing a giant green anaconda he had been tracking.
Danny, Mateo, and Serone search the wreckage of the poacher's boat. An old newspaper photograph reveals that Mateo, Serone, and the poacher hunted together, including snakes. Leaving the poacher's ship, Mateo falls into the water, where the giant male warrior anaconda, measuring 25 ft (7.6 m), ambushes and kills him, while the other two return to their boat, unaware of Mateo's fate. Serone manipulates Gary into helping him ensnare the creature, enticing him with the promise of a $1 million prize. That night, he uses a dead monkey as bait to catch the creature, which proceeds to emerge and attack the crew. He attempts to capture the snake, but it coils around Gary, crushing him. Terri tries to shoot the anaconda, but Serone intervenes, allowing the creature to devour Gary, devastating Denise. The crew overpowers Serone and ties him up as punishment.
The next day, the boat becomes jammed near a waterfall. Terri, Danny, and Westridge enter the water to winch it loose. Denise attempts to kill Serone to avenge Gary's death, but he overpowers and strangles her with his legs, then dumps her corpse into the river. When the creature returns, Westridge distracts it for Terri and Danny to retreat. Serone breaks free during the attack and assaults Danny. The snake mounts a tree branch and attacks Westridge, unintentionally uprooting the tree, which kills Westridge in the subsequent fall. The snake attacks Danny, but Terri kills it by repeatedly shooting it in the head. An enraged Serone boards the boat, attacks Terri, and attempts to kill Danny. Cale, having been awakened by the chaos, stabs Serone with a tranquilizer dart, and Danny knocks Serone into the river.
Further down the river, the survivors discover an abandoned outpost, which Terri and Danny search to find more fuel. Serone catches up to them and ties up Terri and Danny, drenching them with animal blood to bait a second, much larger female queen anaconda, measuring 40 ft (12 m). As it slowly suffocates them, Serone tries to catch it in a net. The snake breaks free and attacks him, eventually swallowing him whole, while Terri and Danny escape their bonds. Terri flees into a building, finding a nest full of newborn anacondas, and is chased up a smokestack. Danny uses a pickaxe to pin the snake's tail to the ground, then ignites a fire below the smokestack, setting the snake ablaze; the resulting explosion sends the burning anaconda flying out of the building and into the water. As Terri and Danny recuperate on a nearby dock, the creature suddenly resurfaces, but Danny stabs its head with an axe.
Afterward, the pair reunites with Cale, who is recovering on the boat. While floating downriver, the trio finds the Shirishama tribe and begins filming their documentary.
