When a diver vanishes in a sudden cave-in, biologist Jason Harville is summoned back to his rural Florida hometown. Jason's estranged brother, Arnie, survives the accident. As teenagers, Jason and Arnie discovered a mastodon tusk in the underwater caves and became national heroes. Jason was inspired to pursue a career in biology, but Arnie succumbed to alcoholism and a life of-poverty. In the aftermath of the cave-in, citizens suffer from lesions and memory loss, the missing diver's body surfaces in a sinkhole not connected to any known underwater tunnel, and the Blackwater River becomes a biological dead zone. Then Jason discovers a prehistoric microorganism that feeds on red blood cells. Triggered by high phosphate levels, the predator's presence indicates illegal dumping. Jason confronts the corrupt town mayor, and with Arnie, returns to the caves, where they discover an underwater mastodon graveyard. This chamber, sealed for thousands of years, ruptured during the cave-in and sent a silent killer into the modern world. They've not only found the origin of the microorganism, but they've solved the mystery of the tusk they found as teens. Jason and Arnie's discovery guarantees the river protection from further pollution and development, and the brothers re-affirm their bond.