Britt Reid (Seth Rogen) is the son of LA's most prominent and respected media magnate and perfectly happy to maintain a directionless existence on the party scene until his father James Reid (Tom Wilkinson) mysteriously dies, leaving Britt his vast media empire. Britt knows next to nothing about his father's newspaper "The Daily Sentinel" or background, and without his father finds himself at a loss with the newspaper's employees, meetings, or even how to manage his own goals. After firing almost all his father's personal servants in the house, he eventually recalls one that he only knows as having been responsible for making his coffee, Kato (Jay Chou). For the newspaper, he hires a new secretary named Lenore Case (Cameron Diaz).
The industrious and inventive Kato had long been working on the prized cars James owned. Finding their love of cars a common ground, the two begin an unlikely friendship. Both agreeing that James had been a man of harsh temper, Britt decides to do something impulsive and settles on decapitating a public statue of his father. With Kato hidden in a car and Britt himself masked with a green cloth, he succeeds, but spots a street robbery taking place. Britt tries to help the victims but finds himself overwhelmed, and Kato sees his employer fleeing from several armed men. Kato quickly analyzes the situation, fights them off and drives a very impressed Britt home. With the incident they see their chance to do something meaningful for the first time in their lives: fight crime.
Britt takes a leading role because of his bolder personality, and the two agree that to get close to the criminals they'll pose as criminals themselves. This way, their enemies would be less likely to suspect their identities or try using hostages to take advantage. Kato, using all his ingenuity and skill, builds advanced retro weaponry including his pride 'Black Beauty', an indestructible car equal parts firepower and horsepower. To spread their fame, Britt leaves the criminals he meets a business card with an email to contact who the press had dubbed 'The Green Hornet', a name Kato had suggested in a meeting with Britt's top press executives. To get an idea of plans and keeping their criminal cover, Britt tells Lenore to research and predict the Green Hornet's criminal behaviours, he himself uses his newspaper to spread news of the infamous new criminal.
Protecting the law by breaking it, Britt becomes the vigilante the Green Hornet as he and Kato hit the streets. Rolling in a mobile fortress on wheels and striking the bad guys with Kato's clever gadgets, the Green Hornet and Kato quickly start making a name for themselves, but tensions soon arise between the pair. Kato is tired of being ignored in the media and being dismissed as the 'nameless thug', and Britt's obsession with his leading role causes him to belittle and mock Kato as the weaker sidekick. As the two grow fonder of Lenore, tensions erupt into a fight that nearly ends with Kato's death, and his dismissal from Britt's employ. Kato finds no sympathy from Lenore, who has also been fired by Britt due to his suspicions about an affair between her and Kato.
Without his usual support, Britt starts to associate with the runner up for District Attorney, Scanlon (David Harbour), a man who had indicated he wanted control of Britt's newspaper in exchange for 'compensation'. Scanlon's uncompromising stance clashes with Britt's temper, and the two leave each other in a rage. Lenore however believes that journalism is all about hearing all sides, so Britt eventually agrees to meet up with Scanlon. What he doesn't know is that an email had been sent to the Green Hornet, and Kato sees that the vigilante had been hired to kill Britt Reid publicly in a restaurant.
At the restaurant, Scanlon tells Britt about his plans to control the media and become D.A., finally admitting that he had been responsible for James's death. Britt shows him a USB he had used to record the entire conversation, but Scanlon is unmoved. As it turns out, Kato in his masked persona had arrived to the restaurant and met with Benjamin Chudnofsky (Christoph Waltz), the man who controls LA's gritty underworld, and the one who had hired him for the assassination. Kato appears in the restaurant and points a gun at Britt's head, scaring the other customers away, but for once Britt does not act recklessly in a tense situation. Finally apologizing for his behaviour to the other man, Britt convinces Kato of his sincerity, and the two turn against Scanlon before Chudnofsky's men appear. The two escape together, and Chudnofsky realizes that Britt himself is the Green Hornet. With Scanlon and many other gang members, they all set out in pursuit of the Black Beauty.
Inside the Black Beauty, Britt assumes his Green Hornet identity while Kato heads for the newspaper offices, intending to upload the recording of Scanlon's confession to the public. Chased on all sides by Chudnofsky's men who all had their own cars and guns, Black Beauty receives a beating but takes the two vigilantes safely to the office, where things descended into chaos as the remaining employees and most of the building's structure and machines get in the way of Black Beauty, Chudnofsky's men, and the police that eventually arrive to the scene. When they finally reach Britt's office, Kato holds off their enemies while Britt tries to upload the data. To his horror, there is no recorded data to be uploaded. He does not stay still for long, after seeing Chudnofsky holding Kato at gunpoint, Britt takes up Kato's mentioned training and clearly analyzes the situation before rushing in to help. Though unable to incapacitate Chudnofsky he does take several mobsters out of commission, and in the distraction Kato blinds Chudnofsky. Before they could take care of Scanlon, the police arrives, and Scanlon quickly identifies himself and tells them to shoot the vigilantes.
Unwilling to let Scanlon escape and let their identities be exposed, the Green Hornet chooses to flee the scene, but not without taking Scanlon down. Crashing what remained of the Black Beauty into Scanlon and out the window, Britt and Kato escape through the injecter seats that, amusingly enough, Britt once jokingly suggested Kato make. Their escape is not completely smooth, for Britt had taken a bullet wound and is unable to go to the hospital without arousing suspicion, so they head to Lenore's home. After beating and pepper-spraying them, Lenore learns their identities and tries to help remove the bullet, but without proper equipment Britt is wary of the procedure. Together, the three of them hatch a plan.
The next day, Britt Reid holds a press conference, claiming that the criminal Green Hornet had destroyed the office in his successful attempt to take Scanlon's life. Black Beauty suddenly bursts into the area and a masked Kato announces that the Green Hornet sends the Reid family his regards, before shooting Britt and fleeing the scene. Lenore accompanies Britt to the hospital to treat his bullet wound, while a police car chases after the Green Hornet's infamous black car. Kato's latest innovation is revealled as the car switches to a white colour to throw off police suspicion, before returning to a black colour once the police car passed.
Sometime later, Britt, in his Green Hornet outfit, arrives at Kato's house saying that they have a secret mission and that everything before was total crap, and that this mission was all that mattered.
The mission: reattaching the head to the statue of James Reid.