The film begins deep in the Carpathian Mountains of Romania during the Cold War, where a group of Soviet and British explorers are searching for a long-lost 13th-century Eastern Orthodox abbey. As they venture inside and below the church, they make a startling discovery: the abbey is built over the entrance to a vast subterranean cave system. When they try to blast their way in, they cause a landslide that buries the abbey and the men are trapped in the cave below it. Once they recover, they continue on into the cave without a choice in hopes of finding a way out, but as they do, they hear strange sounds coming from the darkness.
Thirty years later, a team of archeologists are excavating the ruins of the same abbey, where they unearth images of a medieval battle between the Knights Templar and winged demons, and discover the cave system with its massive underground river. The team is led by Dr. Nicolai (Marcel Iureş) and his associate Katheryn Jannings (Lena Headey), along with her cameraman Alex Kim (Daniel Dae Kim). Local biologists believe the cave could contain an undiscovered ecosystem, so they hire a group of American spelunkers led by Jack McAllister (Cole Hauser) and his brother Tyler (Eddie Cibrian) to help them investigate its unknown depths. Jack and Tyler are thrill-seeking professional cave explorers who run a world-famous team of divers. They arrive in Romania with the latest equipment, including a modified rebreather system allowing a diver to remain submerged for up to 24 hours. The diving team includes rock-climbing professional Charlie (Piper Perabo), first scout Briggs (Rick Ravanello), sonar expert Strode (Kieran Darcy-Smith) and survival expert Top Buchanan (Morris Chestnut).
As they begin the expedition, Briggs is sent to scout and when contact is lost they decide to press on in the likelihood that it's simply an equipment malfunction. After the group finds him safely downriver, Strode is suddenly attacked and dragged away by a large, unknown creature. His water scooter explodes and causes a cave-in, forcing them to follow the river and search for a new way out. Katheryn and Nicolai discover a strange parasite in all of the lifeforms they find. Unlike all the known cave species which have simply adapted over the generations to life underground, Katheryn believes this new parasite originated in the cave environment and has never been exposed to the outside world. As they progress the team occasionally stumble across scattered equipment and remains of previous explorers, and are unaware they are being stalked by the creatures.
They descend through a series of rapids, where Nicolai is attacked and Jack goes after him. Nicolai is dragged into a crevice but Jack breaks free, injured, after seeing letters clearly tattooed on one of the creatures. Jack also begins to exhibit a transformation of his senses, and physical features. When Jack tells them they must go up again to get out, Charlie scales the wall and is attacked by a creature hidden in the passage above. She nearly drops to her death, but recovers, before the man-size winged creature guts her and kills her on the cliff face in full view of the team. Jack's transformations are beginning to show with his super-keen senses and inhumanly slanted pupils. Katheryn speculates that Jack, the previous explorers, and all the rest of creatures mutated because of the parasite. In the case of humans, they resemble demons. As they see Jack changing they suspect his motives and his judgement and the team splits up. Alex, Briggs, and Katheryn decide to go their own way but Top and Tyler stay with Jack.
Jack, Top and Tyler discover a cavern littered with human skeletons and realize that this is the ancient battleground depicted in the abbey above, which sealed the cave to prevent the creatures from escaping. After they see daylight through the underwater passage ahead, Tyler goes back to find the others, but Briggs dies defending Katheryn and Alex, while the creatures enter the cavern and steal the rebreathers necessary to navigate the passage. Alex is killed before they can get in the water, but Tyler, Katheryn and Top escape while Jack stays behind to hold off the creatures.
The three survivors return to civilization, and Top goes his own way. Tyler asks Katheryn if Jack could have survived out in the open. She is quiet a moment then says she had thought that the parasite could only survive underground, but now she is uncertain and thinks that it wants to get out. She bends down to kiss him, looking over the rim of her sunglasses revealing pupils like Jack's as she has begun to mutate. As Katheryn suddenly gets up and walks away, Tyler realizes that Katheryn knows she is infected with the parasite and intends to remain free, able to infect others. He runs after her frantically, but she disappears in the crowd.
The Cave received mostly negative reviews, with a current rating of 11% on Rotten Tomatoes. The consensus on Rotten Tomatoes is, "Despite its stylized and impressive sets, this horror-monster movie mish-mash suffers from endless cliches and wildly implausible plotlines." The main complaint was the lack of character building and overly familiar cliches, with Neil Smith of BBC Online calling it "a generic duffer that is as exciting as a hole in the ground". Robert Koehler made the same observation in Variety, writing, "The Cave feels familiar as it goes through the rote edicts of the scientific crew vs. beastie formula". In her review for the New York Times, Laura Kern praised the films sets and visual style, but denounced the script, writing "Having worked as second- and third-unit director on the Matrix trilogy and Dark City, Bruce Hunt is no stranger to inspired and stylish productions. But whereas those films managed to inject new life into tired territory, The Cave, his first effort as director, fails to generate anything resembling innovation. Judy Chia Hui Hsu wrote in The Seattle Times, "the serenity of the largely aquatic underground world framed by majestic stalagmites and serpentine corridors succeeds in capturing the moviegoer’s attention," but added, "The insipid dialog lacks even a hint of comic relief, so the audience is grateful when the action quickly kicks into gear," and "one of the film’s biggest letdowns is that the vicious beast, seen in the full light of the final scenes, is not such a surprise after all. The creature is simply an amalgamation of monsters we’ve seen before".
In the US, the film opened ranked #5 grossing a poor $6,147,294 at 2,195 sites, averaging $2,800. It went on to have a final US gross of $15,007,991. In Australia, it opened at 89 sites, averaging A$3,204 grossing A$285,121. It had a massive 2nd weekend decrease by 74%, and had a finishing gross of A$570,131. Worldwide, the film has grossed $33,296,457.
The soundtrack was released on August 26, 2005 by Lakeshore Records, which features tracks by heavy metal bands including Atreyu, Lacuna Coil, Diecast, Burning Brides, Ill Niño, Killswitch Engage, Shadows Fall, It Dies Today, Trivium and more. Also, the single Nemo by Nightwish is featured during the end credits of the film.