Allie Fox is an exceptionally talented yet headstrong inventor, deeply frustrated with the American Dream and rampant consumerism. He is convinced that impending nuclear disaster is a consequence of the greed and wrongdoing prevalent in American society. After salvaging parts from a local dump alongside his eldest son, Charlie, he completes his latest innovation—an ice machine he affectionately names Fat Boy. However, when Allie's boss, Mr. Polski, a farmer in the asparagus business, expresses concern about failing crops due to Allie's obsession with his invention, the tension grows. During a meeting, Allie showcases the ice machine, but Mr. Polski remains unimpressed. Allie reflects on the immigrant workers harvesting asparagus, contemplating that to them, ice might be viewed as a true luxury.
The very next day, in a surprising turn of events, Allie hosts a celebration for the immigrant laborers before breaking the news to his family that they are leaving the United States behind. They board a Panamanian barge and encounter several fellow passengers, including Reverend Spellgood, his wife, and their daughter, Emily. Allie soon finds himself in conflict with the Reverend due to their starkly opposing beliefs. Once in Belize City, the families separate, and, in a bold move, Allie buys a small village named Jeronimo from a drunken German.
With the help of Mr. Haddy, their boatman, Allie and his family travel upriver to their new home. There, Allie meets the village's inhabitants and sets to work on creating a new and advanced civilization, inventing various tools and methods along the way. The local residents, including Mr. Haddy, initially welcome Allie and his family, yet Allie's relentless pursuit of his vision pushes them to their limits, provoking discontent. Shortly after, Reverend Spellgood appears again, intent on converting the villagers, resulting in a heated confrontation between him and Allie. This clash cements their ideological divide: Allie sees Spellgood as a fanatic, while the Reverend views Allie as a communist threat.
As Allie's dream of an ice-dependent society progresses, he constructs a more massive version of Fat Boy, which he believes will benefit the town. Inspired by stories of a native tribe that has never experienced ice, Allie gathers his sons to transport the precious resource into the jungle. However, he arrives only to find that their cargo has melted, and the tribe has already been introduced to the concept by missionaries.
Returning to Jeronimo, Allie discovers that the Reverend has successfully frightened away most of the villagers with tales of divine retribution, leaving the settlement nearly deserted. When three rebels arrive demanding refuge, Allie, in a moment of desperation, agrees, intending to devise a way to eliminate them. He locks the rebels in the massive ice machine, hoping to freeze them out, but the plan goes horrifically awry when a gunshot ignites the machine, causing a catastrophic explosion. By dawn, the once-thriving machine and their home lie in ruins, with toxic chemicals contaminating the river.
Drifting downstream, the surviving family members arrive at the shore, with the children and their mother hopeful about a return to the United States. In contrast, Allie clings to the belief that their aspirations are still viable and declares that the U.S. has fallen to nuclear war. He attempts to establish a new life on a houseboat he has constructed, dismissing help from Mr. Haddy, convinced that they’ve achieved a utopian existence. However, a violent storm threatens to sweep them away until Charlie reveals a stash of motor parts given to him in secret, allowing them to power the boat.
As they travel upstream once more, tension builds among the family, stirring resentment toward Allie's unyielding demeanor. Upon discovering Spellgood’s fortified settlement, Allie derides it as a Christian concentration camp. In the dead of night, Charlie and Jerry sneak over to Spellgood's home, uncovering shocking truths: the United States is intact, and Emily will aid them in escaping their father. Just as Charlie makes a desperate pitch to their mother and sisters, Allie reacts by igniting Spellgood's church, leading to a violent confrontation where the Reverend shoots Allie, rendering him paralyzed.
As the family embarks on another journey downriver, with Allie slipping in and out of awareness, he inquires whether they are heading upstream. For the first time, his wife lies to him. The narrative concludes with Charlie's somber reflection on Allie's death, intertwined with a glimmer of hope that the remaining family members may finally embrace their freedom.
