Britt is at his penthouse dictating a letter to Casey before going out to a late dinner with his current girlfriend. They're interrupted when the radio announces the crash-landing of a UFO in a field outside of town. The broadcast reports that several figures fled from the crash before the saucer exploded. They get word from the Sentinel that the citizens are starting to panic as rumors of an alien invasion spread. Before Britt can do anything, four silver-clad figures appear on the balcony in a burst of smoke and break. When Kato tries to subdue them, one of them, a woman, stuns him with a blast of electricity generated from her hands.
The alien leader introduces himself as Mabouse and tells Britt that they are wayfarers. He assures him that Varna, the female alien, only used a stunning charge on Kato and he'll soon recover. Mabouse says that they deliberately sought out Britt so that they could use him to contact the authorities and tell them to leave the aliens alone. Mabouse explains that they want passage to a desert area where his people can pick them up. If Britt refuses, Mabouse will order the sister ships to destroy the city. The alien also knows that Britt has a television transmitter in his penthouse and tells him to make a broadcast reassuring the citizens and calm the panic.
Once Britt goes on the air and addresses the populace, Mabouse tells him to call Scanlon and have him clear a specific route leading out of town. Casey calls Scanlon and Britt relays the message, and Scanlon confirms that the designated route passes by nothing significant. Britt is skeptical of the entire scheme, but Scanlon accepts the aliens' story for now and clears the route. Mabouse departs with his people, taking Casey as a hostage to assure the authorities' good behavior. To prevent pursuit, he has Varna stun Britt and Kato unconscious.
The aliens take a car out of town and rendezvous with a trailer truck. Mabouse takes Casey inside and reveals a combination of high-tech equipment and a luxurious study. She realizes that Mabouse and the others aren't aliens, and Mabouse explains that he was once a scientist, but his peers considered him too advanced and now he seeks power.
Scanlon goes to Britt's penthouse and revives his friends, and informs them that he's confirmed that an Air Force convoy is transporting a nuclear warhead through the area. Britt finally realize that Mabouse is a nuclear scientist that inadvertently caused a near-nuclear disaster a few years ago while developing weapons against American's foreign enemies. Realizing that Mabouse plans to hijack the warhead and fearing the worse, Britt and Kato don their costumes and depart in the Black Beauty.
Using his advanced equipment, Mabouse jams the Air Force convoy's radio so they can't receive a radio warning from Scanlon. The Green Hornet and Kato pick up the jamming signal and realize that Mabouse is responsible. Meanwhile, Casey activates a Hornet tracker, notifying her boss of where she is. She then slips it into the pocket of Shugo, one of Mabouse's men, who is delivering a device that is critical to the scientist's plans.
The convoy arrives in the area ten minutes early and Mabouse is forced to accelerate his plans. He explains that he's going to steal the nuclear warhead and then use the device to arm it. Mabouse offers a champagne toast to Casey, and she throws the alcohol into a control panel. As it short circuits, Casey runs out, gives Mabouse's men the slip, and steals their car. Ashe she drives away, Shugo takes some men and follows her. One of the tires on Casey's car blows out and she's forced to continue her pursuit on foot. She flees to a lumberyard but Shugo and her men capture her.
The Green Horne and Kato have followed the signal to the abandoned cars. The Green Hornet follows while Kato remains with the Black Beauty. The hero arrives at the lumberyard and discovers that Casey has been fastened to a conveyer belt leading to an incinerator. The Green Hornet tries to get to Casey, and Kato arrives to take on Shugo and his men. The crooks flee while the Green Hornet frees Casey just in time.
Varna blocks the road ahead of the convoy and stuns the soldiers when they emerge from the truck. The colonel in charge demands an explanation, and Mabouse has his men attach the arming device to the warhead. The scientist warns that if anyone tries to stop him, he'll detonate the warhead.