A Boston couple, Gail and Tom Hartman, are having marital problems, mostly due to Tom, an architect, spending so much time working. Gail, a history teacher and former river guide, is taking their son, Roarke, on a rafting trip down the Salmon River in Idaho, along with their dog, Maggie. Their daughter, Willa, is staying behind with Gail's parents in Idaho. Tom, who had remained in Boston, unexpectedly joins them at the last minute. As they are setting off, they meet three other rafters, Wade, Terry, and Frank, who appear to be friendly.
The Hartmans catch up with the trio during a day break, and notice that Frank is no longer with Wade and Terry. They explain that he hiked out after an argument. Unfortunately, he was their guide, and Wade and Terry lack any rafting experience. Gail offers to guide them down the rest of the river. Before getting back on the water, Maggie wanders off and becomes curious about something in the brush farther up the canyon. Tom fetches her before she uncovers it, and they return to the raft.
After a day's rafting, they make camp for the night, but Tom continues working on his architectural project, disappointing Roarke, who feels neglected. They are joined by Wade and Terry, who help celebrate Roarke's birthday that night. After Wade begins acting suspiciously, Gail agrees with Tom that they should part ways with him and Terry. Their plans are upended when Wade and Terry shove off first with Roarke aboard their raft. Wade, showing off to Roarke, reveals they have a gun. During a rest stop, Gail and Tom attempt to take off with Roarke before Wade and Terry notice. That fails, and Wade pulls the gun on Tom. As they struggle, Maggie runs off into the bushes. Gail then realizes that Wade and Terry committed a recently reported robbery and have killed Frank, who was wounded during the robbery and was slowing them down.
The Hartmans are forced down the river at gunpoint before setting up camp for the night. During the night, Tom tries and fails to wrestle the gun away from Terry. Tom runs into the river with Wade chasing him. Wade shoots at Tom and appears to have hit him. Wade returns telling Gail and Roarke that Tom is dead. It is then revealed that Wade’s gunshot had missed Tom, and he had escaped. The next day they run into a ranger, Johnny, who knows that Gail intends to run the Gauntlet, and warns her not to try. Wade shoots him and throws his body into the rapids.
Unbeknownst to anyone, Tom, who finds Maggie, is racing on foot along the canyon rim to get ahead of the raft. After a harrowing ride, the group makes it through the Gauntlet. Tom reappears, and flips the raft. As he struggles with Terry, Gail is able to get the gun. She shoots and kills Wade while Tom subdues Terry. A helicopter with rangers aboard arrives, and they arrest Terry. Gail and Tom share a kiss by the rapids. The film ends with the Hartmans in embrace.
