The Phoenix Foundation is hosting a series of student environmental exhibits at the Challengers Club. MacGyver is handing out fliers because his friend Breeze hasn't shown up yet. He gives Breeze a call, and the teenager informs him that he had to work overtime at the garage, but he's on his way.
A teenage junkie, Todd Fowler, tries to trade a gun to dealers Rudy and Duke in return for some crack. Rudy takes the gun but the police see the exchange and order Rudy and the others to surrender. Rudy shoots one of the officers. He and Duke escape in a car while Todd manages to slip away, unseen.
Rudy and Duke lose control and crash on the streets as Breeze is walking by. Breeze recognizes his former friend, Rudy, and tries to stop him. Rudy and Duke run off and the police arrive and think Breeze was involved. Breeze panics and runs to the garage where he works, and calls MacGyver for help while the police surround the building. MacGyver gets there and refuses to let the police open fire. He convinces Breeze to surrender and promises he'll stay with him.
Later, a Middle Eastern arms dealer, Zamora, is watching a news broadcast about the shooting. Dr. Fisher, a forensics expert, has learned that the gun used in the shooting was the same one used to kill a presidential candidate, Senator David Harrison, twenty years ago. Zamora calls his American supplier, gun manufacturer J.D. Maddox, and tells him to recover the gun. Maddox doesn't understand why and Zamora explains that he can use the gun to inspire a new wave of anarchy and terror. He tells Maddox to get Breeze out of jail and force him to turn over the Harrison gun, or he'll cut off payment on the arms shipment. Maddox reluctantly agrees to recover the gun.
Pete and MacGyver check the history of the Harrison gun and learn it was a rare Czechoslovakian seven-shot Ruger. It has a history of violence, almost as if it's cursed. After it was recovered following Harrison's death, it was sold at police auction to a gun dealer. It was then stolen and used in a drive by shooting, and a stray shoot killed an innocent woman. It was recovered and sold again at police auction where a man named Jenkins bought it and the trail disappears.
Laura Fowler returns home and sees a newspaper article on the Harrison gun and the fact it's reappeared. She realizes that it's missing from a gun case in her home. She goes to talk to her brother Todd in his room and discovers him smoking crack. She says he broke his promise to their father and asks if he used the gun Todd denies it but then realizes that he sold the wrong gun by mistake and goes to find it.
MacGyver learns that Jenkins died in a car crash and his family sold the gun. There's no record of who purchased it. MacGyver goes to pay Breeze's bail thanks to the Foundation but learns that Maddox has already posted bail. He goes to Maddox's factory and bluffs his way past the security guards, and finds Maddox and his men talking to Breeze on the factory floor. Despite Maddox's offer of $1,000, Breeze insists he doesn't know where the gun is. He tries to leave and Maddox's men start to get rough, but MacGyver arrives. Maddox dismisses him as a bleeding-heart liberal who doesn't believe in the Second Amendment, but MacGyver points out that Maddox isn't selling guns for the purposes of a well-regulated militia. He collects Breeze and they leave.
Outside, MacGyver demands to know the truth and Breeze explains that he went to camp with Rudy when they were young. It was at camp that Breeze fell in love with environmental causes and saving the planet. He doesn't want to turn over Rudy and betray a brother, but MacGyver insists that he owes it to the people who have lost their lives to the Harrison gun.
Rudy and Duke read about the Harrison gun in the paper and Rudy tells his partner that he threw it down the sewer. Duke goes to get it and Rudy slips away to a nearby wrecking yard. Todd is watching him and Laura arrives to confront him. Todd ignores her warnings and goes after Rudy, and MacGyver and Breeze arrive and spot Laura as she drives away. At the wrecking yard, Rudy recovers the gun from the car where he hid it and Todd attacks him. The two struggle and Todd accidentally shoots Rudy. MacGyver and Breeze hear the gun shot and run after Todd. The junkie knocks out breeze and escapes, and MacGyver and Breeze discover that Rudy is dead.
Pete confirms that the Harrison gun was the weapon that killed Rudy. MacGyver goes through newspaper reports and discovers that Laura is the daughter of Frank Fowler, a gun-control advocate who created the Harrison Foundation for Gun Control.
At his house, Todd calls the weapons plant and talks to Maddox's henchman, Wyatt. He offers to sell the gun and promises to call back to talk to Maddox in a few minutes. MacGyver arrives out front and Todd recognizes him from the wrecking yard. He tells Laura to cover for him and hides in his bedroom.
Wyatt tells Maddox about Todd's call and shows him the phone number on their caller ID. Maddox calls to tell Zamora that he can pick up the gun at the factory at five o'clock along with his arms shipment.
MacGyver and Breeze explain to Laura that they're looking for the Harrison gun while Todd secretly listens in. MacGyver notices an empty gun slot in the display case and Laura explains that her father and Senator Harrison were close. After Harrison died, Fowler started the gun control foundation and bought crime guns to take them off the street. He manages to obtain the Harrison gun and kept it at home with the others. Todd makes a noise, alerting MacGyver and Breeze, but they lose him on the streets. Back at the house, they discover that Maddox's men have traced the phone number. The thugs take MacGyver, Breeze, and Laura prisoner.
Zamora arrives at the factory to pick up his shipment and the Harrison gun. Todd calls and Maddox tells him that he should bring the gun to the pier at in an hour and he'll pay for the weapon, and then turn it over to Zamora. The thugs bring in MacGyver, Breeze, and Laura, and Zamora realizes they can identify him. He tells Maddox to put them in a cargo container with his shipment and he'll dump them out to sea. As they're taken to the piers, MacGyver searches the crates and finds grenades and laser sights.
When MacGyver doesn't check in, Pete decides to check things out himself. He goes to the Fowler house as Todd arrives to recover the gun from where he hid it in the sofa. Todd drives to the pier and Pete follows him. When he arrives, he turns over the gun and Maddox's men knock him out. Pete sneaks up and knocks out Wyatt, saving Todd's life.
As they're lifted into the air, MacGyver attaches the C-4 from the grenades to the spot where the forward support cable passes through the cargo container. He and the others strap themselves into the back end of the container and then MacGyver uses the battery from the laser sight to set off the explosives. The front end of the cargo container drops down and the crates drop onto Zamora and Maddox, killing them.
Later, the Phoenix Foundation starts a new initiative to destroy crime guns so that they're not sold at public auction and put back onto the streets. After the chief of police speaks, Breeze gets up and talks about how he fell in love with the environment and he plans to help by getting rid of guns. Finally, Laura donates her father's collection of crime guns and places the Harrison gun into the vat to be melted down. Pete notes that they've made a good start, but MacGyver warns that they have millions more to go.