Jeb Maynard (Charles Bronson) is head of the US Border Patrol office in eastern California, near Arizona. Hotchkiss (Ed Harris) is an ex-marine, getting rich smuggling people across the border. A border patrol agent (Wilfred Brimley) is shot dead by Hotchkiss. The FBI is sure it was a drug crime, but Maynard is convinced it was a human trafficker, and wants revenge for his friend.
Maynard tries several tactics to track the killer, even crossing into Mexico, pretending to be an illegal alien, and hiring the traffickers to bring him back into the US. He is getting nowhere. Jimmy Fante (Bruno Kirby) is an agent transferred from NY. Maynard is skeptical of his scientific means of solving crimes. Many tomatoes had fallen from the truck when Brimley was killed. Fante gets testing on the tomatoes, and finds that only 6 farms raise that variety of tomato and use that pesticide. Maynard and Fante visit all 6 and find Hotchkiss's base.
The traffickers lie to the immigrants, and actually sell them into forced labor. They have a big order from the NY mob, and need to move a huge group across the border. They plan to do it Christmas eve, thinking border patrol will be light. Maynard arrests one of Hotchkiss's drivers and "forcefully interrogates" him. They learn all the plans. Border patrol agents swarm the farm on Christmas eve, and arrest everyone on each truck as it arrives. Hotchkiss is in the last truck, but realizes something is wrong. Hotchkiss escapes on foot, heavily armed. Maynard uses old school techniques to track him through the woods. In their confrontation, Maynard shoots and kills Hotchkiss.
The epilogue shows the sentences Hotchkiss's gang gets, most of which are very light. The NY mobster, who was buying the slave labor from Hotchkiss, gets no jail time.
Thanks Joseph C!