Vince Majestyk (Charles Bronson) is a farmer, an ex-con, a former U. S. Army Ranger instructor and Vietnam War veteran, who owns and operates a watermelon farm in rural Colorado. He needs to harvest his crop soon in order to keep the farm financially solvent.
A small-time hood, Bobby Kopas (Paul Koslo), attempts to coerce Majestyk into a protection racket of using unskilled drunks to harvest his watermelon crop. Majestyk runs him off with Kopas’s own shotgun and hires experienced Mexican migrant workers, including Nancy Chavez (Linda Cristal), a crops picker who is also a union leader. They are romantically attracted to each other. Kopas brings assault charges against Majestyk, resulting in the farmer being placed under arrest before he can finish the harvest.
In jail, Majestyk meets and annoys Frank Renda (Al Lettieri), a notorious mob hit man being transferred to a higher-security prison. Renda’s men try to break him out of police custody during a prisoner transport by bus. In the escape attempt, Majestyk drives off in the bus with Renda still in handcuffs, eventually taking him to his hunting cabin in the nearby foothills. Majestyk hopes to trade Renda to the police in return for being released to finish harvesting his melons. Renda offers his captor $25,000 for his freedom, but Majestyk declines. Renda then threatens to kill Majestyk if he doesn’t release him, and Majestyk pretends to be persuaded to take the money, but contacts both the police and Renda’s mafia contacts to come pick them up.
Wiley (Lee Purcell), Renda’s girlfriend, arrives and they manage to turn the tables on Majestyk, although he is able to escape. Renda learns the charges for which he had been imprisoned have been dropped. He meets up with his right-hand man, Lundy (Taylor Lacher), who advises him to fly to Mexico and enjoy himself; Renda will have none of it…he wants revenge on Majestyk. He arranges for Kopas to drop the assault charges against Majestyk, and orders his men to find the “melon picker” so he can have the satisfaction of killing him personally.
They arrive at the farm to kill Majestyk, but not finding him they instead machine-gun the melons and rough up the hired hands, forcing them to depart. The next day, Kopas badly injures Majestyk’s friend as he tries to deliver a load of melons, putting him in hospital. As Majestyk and Nancy have a drink at a bar in town Renda approaches and tells him he plans to kill him. Instead of being intimidated, Majestyk knocks Renda down, telling him to ‘call the cops’, further infuriating Renda.
Renda and his men surround Majestyk’s home, but Majestyk gets away in the back of a pickup truck driven by Nancy, and a prolonged car chase ensues. The police set up roadblocks and launch a helicopter hoping to find them. Luring Renda and his men into the foothills, Majestyk turns the tables on them and becomes the attacker, killing most of Renda’s men during the pursuit. Realizing they are now the hunted, Renda retreats to their lodge hideout where he, Wiley, Lundy and Kopas hole up. Majestyk arrives and disables their vehicle. Renda sends Wiley outside to negotiate with Majestyk, hoping to force him to show himself, but Majestyk sends her away with Nancy. Majestyk then assaults the cabin, killing Lundy after Renda sacrifices him to save himself. Disgusted, Kopas decides to leave when he realizes Renda will also sacrifice him to get to Majestyk. With Kopas’ help, Majestyk gets the drop on Renda and kills him. The police soon arrive and arrest Kopas and Wiley, while Majestyk drives off with Nancy.