Dave Bannister asks DA Frank Scanlon and Detective Olson to attend the funeral of his father James, with a promise to explain afterward. As the two men watch the funeral from 6their car, they're unaware that one man has taken out a strangely-shaped gun from his pocket. He fires it at Dave and the gun makes no sound or flash. Dave simply topples over, dead, and Scanlon and Olson run over and confirm that someone shot him.
At the Daily Sentinel, Britt is reading an article on Dave's murder and how he was killed in front of twenty witnesses who didn't hear or see a thing. Scanlon calls Britt on a scrambled line and asks for a meeting, explaining that he has the coroner's report on Dave's murder. Ballistics show that Dave was killed with a seventeen caliber slug, something that doesn't exist, and Britt agrees to meet with his friend. Reporter Mike Axford bursts in and tells Britt that he has a lead on Dave's girlfriend, Jackie Cameron. She called to say that she knows who killed Dave and wants $2,000 for the information so she can get out of town. Britt agrees to allocate the funds and Jackie calls. She agrees to come to the Sentinel that night to get the money and tell her story.
Britt drives to his penthouse apartment while Scanlon comes up through a secret entrance in the alleyway. Kato, Britt's valet, is waiting with Britt to meet with the DA. Scanlon explains that the OSS used a twenty-two caliber silent, flashless gun in the war, but they're all accounted for. However, Dave's father James was a retired gunsmith who could have built such a weapon. Britt tells Scanlon about Jackie's offer. As they discuss the case, Britt's secretary Casey calls to tell them that someone killed a brokerage house messenger in a crowded elevator using the same weapon. No one heard or saw anything and the killer escaped with $5,000 in bearer bonds. Britt and Kato tell Scanlon that they'll look into it and then go to the garage, donning their costumes as the Green Hornet and his chauffeur. They drive out in the Black Beauty and hit the streets.
That night, Mike meets Jackie at the Sentinel and pays her the $2,000. However, the Hornet and Kato burst in and gas Mike unconscious. The Hornet then pays Jackie $4,000 for the name of Dave's killer, assuring her that she'll be under his protection. He figures that she's dirty because she went to the newspaper instead of the police, and tells Jackie that he wants the gun for himself. Convinced, Jackie tells the heroes that one of mobster Al Trump's thugs, Joe Renner, used the gun to kill Dave. Dave tried to set aside money for their marriage and gambled and lost $2,000. He was in debt to Trump and got him the silent gun that James built to pay off his IBUs. When James found out about it, Trump had him killed in a hit-and-run and then killed Dave when he threatened to go to the police after his father's death. Jackie tells the Hornet that he can find Renner at the Argyle Arms and then heads out of town.
The Hornet and Kato go up the side of the apartment building on the fire escape and spot two thugs emerging from Renner's room. They work for Dan Carley, Trump's rival gangster. The heroes go into Renner's room and find the man dead on the floor. They figure that Carley's men took the silent gun but before they can leave, Trump and two of his men come in. They figure that the Hornet killed Renner and try to call the police. Kato distracts them with a well-placed kick and the two vigilantes take down the mobsters. The Hornet tells Trump that Carley's men were there and threatens Trump until he admits that he still has the gun. The vigilante offers to negotiate a deal between the two mobsters: Carley pays Trump for the gun, Carley calls off his attacks on Trump's men, and Hornet gets half of the payoff. Trump reluctantly accepts the deal rather than face the combined wrath of Carley and the Hornet.
Later, the Hornet and Kato break into Carley's study and make short work of his men. The Hornet offers Carley the deal, but Carley wants a demonstration of the gun before he pays for it. The vigilante agrees to set up a demonstration, while Kato secretly places a bug beneath the table. Once Carley agrees, the two heroes leave... and Carley calls Trump. The Hornet and Kato record the conversation. When they get back to Britt's penthouse, they play the tape for Scanlon. Carly offers to make a deal with Trump to sell out the Hornet at the demonstration, eliminating the competition and saving themselves the money they're paying him to broker the deal. Britt figures that they'll have to play along so they can get the gun and tells Scanlon that the Hornet started it so he'll finish it without the police.
Later, Trump picks up the Hornet and Carley and drives them to an auto garage that he owns. Kato follows them using a tracking device on the Hornet. A police car spots the Black Beauty and gives chase, but Kato easily loses them after listening in on the police scanner.
At the garage, Trump gives Carley the gun and the mobster prepares to test it. Realizing that the two men are making their move, the Hornet shoves them down and runs down the backstairs. He reaches the main garage, sabotages the lights, and hides among the cars. When Trump and Carley come in, they lock the doors and split up to look for the Hornet. The vigilante blinds Trump with an oil gun and knocks him out, but Carley gets the drop on him.
As Carley admits that he has to kill someone that is almost his intellectual equal, Kato blasts a hole in the wall with the Black Beauty's cannons. Carley shoots but misses, hitting a steam pipe that sprays hot steam into his face. He collapses, unconscious, while the Hornet has Kato call Scanlon to pick both mobsters up.
Later, Britt publishes the story and Mike admits that he couldn't find out how the police recovered the silent gun. He does find the idea of the gun in the hands of a master criminal like the Green Hornet frightening, and Britt and Casey agree.