The L.A. District Attorney is named Robert Caulfield, played by Gene Hackman, and the witness is a woman named Carol Hunnicut, played by Anne Archer. Carol is stubborn and initially refuses to cooperate, but eventually agrees to testify against the mob boss.
Caulfield and Carol board a train in Seattle bound for Vancouver, where they plan to catch a flight back to Los Angeles. However, they are followed onto the train by two hitmen, who are determined to kill Carol before she can testify.
Caulfield and Carol must evade the hitmen while also dealing with other passengers on the train who may or may not be involved in the assassination plot. Among these passengers is a friendly but mysterious man named Scott, played by James Sikking, who offers to help Caulfield and Carol.
As the train travels through the wilderness of western Canada, Caulfield and Carol must use all their wits and resourcefulness to stay one step ahead of the hitmen and figure out who can be trusted. Along the way, they encounter various obstacles, including a dangerous avalanche and a train derailment.
In the end, Caulfield and Carol are able to outsmart the hitmen and safely make it to Vancouver, where Carol is able to testify against the mob boss. The film ends with a scene of Caulfield and Carol sharing a moment of relief and gratitude on the steps of the courthouse.