Immigration officers engage in a gun battle when smugglers try to unload a boatful of illegal immigrants onto a U.S. beach. Later, at the U.S. Immigration Bureau, an officer complains to his superior that although illegal immigrants, or "wetbacks," are being smuggled across the borders of Texas, California and Arizona, the bureau has no idea where the operation is based. Describing the smugglers as "the most vicious kind of human scavengers," the official remarks that they take in four to five thousand dollars per week from families who often give up their life savings in order to get one of their members into the United States. Once there, the immigrants are hired out as farm labor, while the smugglers, who have promised their customers good jobs and homes, simply disappear. The official, however, has a plan. Jim Benson, an ex-Coast Guard officer and present owner of a nearly bankrupt charter fishing business, persuades the man to whom he is deeply in debt to let him take his vessel on one more trip. As Jim tears the "Sheriff's Sale" notice from his boat, two men, one of them Mexican, approach him with a business offer, but he ignores them. At that moment, Karl Shanks and Sally Parker, Jim's obnoxious customer and his date, arrive for their Mexican fishing trip. While Jim makes coffee, Karl quietly tinkers with his fishing rod, and during the trip, complains constantly. Finally, he offers Jim twice his fee if he can locate a fish, but threatens to pay nothing if they return from the trip empty-handed. A huge marlin bites, but because Karl has tampered with the rod, the fish gets away. Realizing he has been tricked, Jim threatens to strangle Karl if he refuses to pay. Karl demands to be taken to the nearest town, promising to give Jim the money. When they arrive in Delgado, Mexico, however, Karl secretly pours beer in the gas tank and runs away. The two men from the dock, who described themselves as importer/exporters, again try to hire Jim, but he is suspicious of them and refuses. Alphonso, the owner of the local cantina and inn, has neither the gasoline nor the money to help his stranded friend, and even Sally, sitting dejectedly at the bar, declares that she is broke. Jim and Sally talk until late, whereupon she retires and Jim returns to the boat. The two men, who identify themselves as Steve Bodine and Juan Ortega, admit that they want Jim to carry a number of Mexican men across the border, claiming that the work that awaits them there will give them and their families "a chance." Because he so desperately needs the money, Jim reluctantly agrees to make the run, but when Sally questions him, he changes his mind. Bodine and Ortega, however, threaten Jim with a gun and begin to load men onto the boat. The wife of one of the laborers, not believing that her husband will be paid a fortune just for picking cotton, begs him not to go, but he insists on making the trip. During the run, Jim tries to change course, but Bodine's henchman notices and punches him. A Coast Guard vessel calls to Jim, but because the crew knows him, they allow his boat to pass. When Jim returns to Delgado, Bodine informs him that he must continue making the runs. Frustrated and scared, Jim confesses to Sally, but when he refuses to admit his crime to the police, she reveals that she is a U.S. Immigration Bureau agent. Jim reacts with fury to the news that he has been planted in Delgado to help uncover the smugglers. They argue, and Sally leaves, but Ortega sees her peering into the smugglers' headquarters and locks her up. Back in the U.S., an Immigration inspector learns that Jim is being used and angrily orders his men to get him out. Meanwhile, Sally frees herself and finds Jim in Delgado's main street, where they hide temporarily from the smugglers by blending in with the crowds of a local fiesta. Jim's little friend Pedro secretly siphons gas from the town taxi and pours it into the tank of Jim's boat. The smugglers beat the boy, but Jim and Sally escape to the boat, which has been loaded with a single immigrant. Their vessel is closely pursued by the smugglers, and while hiding with Jim in a small cove, Sally explains that she became an agent after her father, a boatman, was killed by wetback smugglers. Bodine and Ortega chase the couple through the waves, and Jim tries to take cover from their bullets by weaving around a large freighter. The arrival of a Coast Guard boat sends the smugglers running, but they are soon caught. Shanks reveals that he, too, is an agent, and that he has deposited a large sum of money in Jim's account. On Jim's boat, Sally and Jim kiss.