At the Challengers Club, the teenagers are holding a science fair on environmental issues. MacGyver is filling in for Breeze, who has gone missing. MacGyver goes to give him a call and reaches him at his home. Breeze explains that he had to work overtime and is on his way. He leaves his home on the way to the club.
On the street, a junkie, Todd Fowler, tries to sell drug dealers a gun to pay for his fix. When the police came by, the drug dealers run for it, shooting one officer with the gun. They take off, leaving the junkie behind. He slips away while the police close in on the dealers. They drive by Breeze and slam into a car. Breeze recognizes one of them as a former friend, Rudy Gibbs. Rudy runs off and the police tell Breeze to hold it. He runs off, panicked, and breaks into the garage where he works. Breeze calls MacGyver and asks him for help.
MacGyver arrives as the police surround the building. They refuse to listen to him so he gets between the police and Breeze, insisting that he doesn't have a gun. MacGyver calls upon Breeze to come out and surrender. MacGyver promises he'll stay with him.
Later, Zamora is listening to a news broadcast about the shooting. The bullet that killed the officer came from the same gun that was used 20 years ago to kill a presidential candidate, David R. Harrison. Dr. Fisher, who worked on the task force to solve the shooting, made the match and realized the guns were the same unusual model. Zamora calls a gun manufacturer, J.D. Maddox, who is shipping him a major gun purchase and tells him to recover the gun no matter what it takes. Maddox gives in and Zamora tells him to get Breeze out of jail and force him to tell where he got the gun. Zamora explains that the gun symbolizes anarchy and could be used as a rallying point for desperate men, and increase gun sales. Maddox tells his men to get to Breeze.
Pete and MacGyver check the records on the Harrison gun and learn that it was a rare Czechoslovakian seven-shot Ruger. It was sold at a police auction after the trial to a gun dealer and stolen a few weeks later in a robbery. Six years ago, a woman was killed by a stray bullet from the gun in a driveby shooting. The Harrison gun was sold at auction again to a man named Jenkins and then disappeared.
As Laura Fowler returns home, she sees an article on the Harrison gun. She goes to a gun case and sees that a slot is missing. Laura goes to talk to her brother Todd and discovers him smoking crack. She accuses him of breaking his promise to their father and he tells her to leave him alone. Laura asks where the gun is and if he killed the policeman, but he denies it at first. When he realizes what has happened, he explains that he sold the wrong gun by mistake. He runs out to get it.
MacGyver discovers that Jenkins was killed in a car crash and sold the gun, but there's no record. Pete lets him know that the Foundation has posted bail for Breeze. MacGyver goes to pick him up but discovers that Maddox with Maddox Weapons has already posted Breeze's bail. MacGyver drives to Maddox's factory and bluffs his way past the security guard. He spots Maddox talking to Breeze, offering him $1,000 for the Harrison gun. Breeze says he doesn't know about it and tries to leave. When Maddox's men stop him, Breeze spots MacGyver and Maddox lets him through. Maddox defends himself, citing the Second Amendment, but MacGyver isn't impressed. Maddox reluctantly lets them go. Outside, MacGyver wants to know who shot the cop and doesn't believe Breeze's denials. Breeze refuses to turn over someone he knows and insists it isn't fair, but MacGyver points out the people who lost their lives to the gun didn't think it was fair, either. Breeze admits he was protecting Rudy, a pusher, and explains that they went to camp together in the country. That's when Breeze got hooked on saving the environment.
Rudy sees the article on the paper about the Harrison gun and claims he threw it down into the sewer. His partner Duke says he'll try to find the gun and cash in. Todd watches them and Laura comes after him. When Rudy takes off, Todd abandons his sister and follows the pusher. Breeze and MacGyver pull up nearby, looking for Rudy, and MacGyver spots Laura as she drives away. Meanwhile, Todd follows Rudy into a wrecking yard and watches as Rudy recovers the gun. The two struggle and Todd inadvertently shoots Rudy. Breeze and MacGyver hear the gunshot and run to investigate. Breeze gets there first and tackles Todd, who knocks him out with an auto part and flees. MacGyver and Breeze go back to check on Rudy, who tells them that Todd has gun before he dies.
At the Foundation, Pete gets the ballistic report and confirms the Harrison gun was responsible. MacGyver identifies Laura as the daughter of a famed gun-control advocate, Frank Fowler, who worked for the Harrison Foundation against guns.
Todd calls Maddox and gets one of his men, Wyatt. He says he has the Harrison gun and promises to call back in a few minutes. MacGyver pulls up outside and Todd recognizes him from the wrecking yard. He asks Laura to cover for him and slips out. At the factory, Maddox's man tells his boss that they have the phone number on redial and can trace the address. Maddox tells Zamora that he'll have the gun at 5 o'clock and his client can get the gun and his shipment at the same time.
Todd listens as MacGyver and Breeze talk to Laura and explain they're looking for the Harrison gun. She asks them to leave but MacGyver notices it's missing from the display case. Laura finally explains that her father loved Senator Harrison, and took the tragedy hard when the senator was killed. Frank eventually bought the gun and stored it, believing it was cursed. Todd bumps a picture, alerting MacGyver and Breeze, who chase after the teenager. Todd runs for it and manages to get away. They go back to the house to see if Laura can tell them where Todd hangs out, but Maddox's men are there and capture them all.
At the factory, Zamora demands to know where the gun is. Todd calls and Maddox tells him to meet them at Pier 9 in an hour. Maddox then tells Zamora that he can get the gun at the freighter along with shipment. Zamora tells Maddox to put MacGyver, Breeze, and Laura into a cargo container and send them to his boat, and he'll dispose of them at sea.
When MacGyver doesn't check in, Pete gets nervous and goes to investigate.
Locked in a cargo container en route to the pier, MacGyver goes through the crates and finds grenades. He finds a rifle with a laser sight and starts taking it apart.
Pete arrives at the Fowler home as Todd comes home to find his sister. He takes the gun from its hiding spot in the sofa and leaves, and Pete spots him and follows.
The semi arrives at the pier and MacGyver notices the winch cable above passing through a hole in the ceiling.
Todd arrives at the pier and MacGyver and Laura spot him as he meets with Maddox. Maddox's men knock him out and then load the cargo container onto the ship. MacGyver finishes rigging the explosive and has Laura and Breeze strap themselves into the back of the container.
Pete approaches the freighter and knocks out Wyatt, then finds Todd and asks where MacGyver and Breeze are.
MacGyver attaches the explosive to the winch cable and straps himself in with the others. He activates the charge and the container flips over, dumping its contents onto Zamora and Maddox and killing them just as Maddox hands over the gun to his buyer.
Later, the Foundation is holding a ceremony to destroy guns by melting them down, rather than selling them at auction. MacGyver, Pete, Breeze, and Laura look on as Chief Cooke speaks, saying that while there are legitimate uses for guns, they don't need to be sold back into the community by the police. Breeze steps forward to talk about the environment and how they need to get rid of another kind of pollution: guns. He introduces Laura, who donates her father's collection of crime guns. She turns over the Harrison gun to be melted down into slag.