While waiting to leave a small town in Montana, Dr. Kimble is spotted by the authorities and arrested. He is cleared for having stolen a truck but the sheriff decides to jail him for having resisted arrest and suspects he's not who he says he is. While Kimble is held in jail, another prisoner determines his true identity.

Kimble is blackmailed into slaving for a physician obsessed with curing a deadly strain of meningitis prevalent in a remote Mexican province. The fugitive put ashore in Mexico, after fleeing U.S. police, and swapping places with a departing sailor. The expatriate Dr. Howell will stop at nothing to wipe out his "tiny enemy," but Richard Kimble contests the ethics of Howell's fanatical methods, as a wily local police sergeant closes in on the convicted killer.