At the airport, a plane full of passengers sits, waiting for its air conditioning to be fixed. When the repair truck arrives, one passenger gets out of his seat and taps the pilot at the exit hatch on the shoulder. While the repairmen plug a huge tube into the plan, the pilot closes the hatch. Inside, the passenger who left his seat puts on a gas mask just as green smoke begins to come through the AC vents. The passenger who had been sitting next to him does the same. Before long, the other passengers are unconscious and the two gas mask wearing men cut open the briefcase of one of them, stealing a diamond necklace. Before they leave with their companions, the repairmen and pilot, one of them places a sticker with the emblem of the Green Hornet on the briefcase.
Later, at the Daily Sentinel, Britt Reid and Mike Axford watch a news report goes out about the robbery. According to the report, early police indications lead to the Green Hornet being behind it. Mike believes that the Green Hornet is the cause of the robbery because his seal was left behind at the scene. Britt cautions that anyone could have left that to point toward the Green Hornet. Suddenly, Casey comes in with a letter for Britt. After dismissing Mike, and assuring Casey that he hasn't taken the Black Beauty out all week, Britt opens the letter. Inside, its writer claims to have predicted that the Green Hornet would rob the plane, a full 16 hours before it happened. He also claims to be able to pinpoint the Green Hornets next target.
At the lab of the letter writer, Dr. Abel Marcus, Britt asks him why use his computerized crystal ball to predict the Green Hornet's moves. Marcus says it is because the Green Hornet is the most masterful criminal of their time. Because he knew Britt was coming, Marcus had his assistant, Laura Spinner, enter some calculations. As Britt stands by, Laura flips some switches and turns some dials on console and the next prediction comes out. The Green Hornet will strike that night at 9 o'clock at the city sports arena box office.
Before leaving Marcus's lab, Britt sends out a signal to D.A. Scanlon. Meeting him at his house, Scanlon asks Britt about what is going on. Britt shows him Marcus's letter. Scanlon is ready to go arrest Marcus, but Britt stops him because Laura Spinner is actually doing the technical work. Britt says he found out she is one of the top computer technicians in the country. Britt then tells Scanlon about the sports arena being the next target. Scanlon is ready to mobilize, but Britt tells him to let Marcus string out some more rope for his noose.
Later, the Green Hornet and Kato race to the City Arena in the Black Beauty. They arrive 15 seconds before the predicted robbery time. Seconds later, a car carrying three men in gas masks pulls up outside the arena. The masked men rush inside, take the money from the box office attendant and gas him. Before leaving, they place a Green Hornet seal on the box office. The Black Beauty gives chase as the robbers flee the scene, but must abandon the search when a police cruiser begins chasing them. Losing the cruiser, Green Hornet and Kato head to Marcus's apartment, where the find the car from the robbery parked outside.
Climbing on a ledge outside, Green Hornet and Kato spy a costume party in Marcus's apartment. Crashing through his skylight, they make their presence known. Marcus is amused at first, believing them to simply be fellow party goers, but the Green Hornet soon proves that he is no fake.
Going into a back room, Green Hornet questions Marcus about what he is up to. Marcus says that he and the Hornet are now 50/50 partners. Green Hornet questions why he would need a nickel and dime (small time) operator like Marcus for a partner. Marcus begins to respond, but Green Hornet cuts him and asks where the small predictions are leading. Marcus says that he will next rob Barker and Benton Stock Brokers of a quarter of a million in negotiable securities. To deter the police, he will predict the Green Hornet's next target to be the First Metropolitan Bank. He says a fan will make sure the police believe him.
The next day, in Britt Reid's office at the Daily Sentinel, Marcus tries to convince Britt to force the police into believing his prediction about the bank. Britt agrees and later, while discussing how to catch Marcus with Scanlon, gets a call from Casey. Marcus says his computer has contradicted his earlier prediction and listed Barker and Benton as the next target.
Meanwhile, Laura stumbles onto Marcus's real plan: to steal $3 million in gold from the waterside Customs warehouse. Subduing the guards and collecting the gold, Marcus and his gang nearly escape, when the Black Beauty roars into the warehouse. Green Hornet subdues some of the henchmen on his own, while Kato deals with the others. In a fight on some storage crates, Green Hornet defeats Marcus. Later, it is revealed that Laura's testimony will ensure the conviction of Marcus and his cohorts. Mike remarks that he knew Marcus was a bad guy all along, a claim that Casey and Britt just smile and agree with.