In the future of 2026 (100 years after the film was produced), wealthy industrialists rule the vast city of Metropolis from high-rise tower complexes, while a lower class of underground-dwelling workers toil constantly to operate the machines that provide its power. The Master of Metropolis is the ruthless Joh Fredersen, whose son Freder idles away his time in a pleasure garden with the other children of the rich. Freder is interrupted by the arrival of a young woman named Maria, who has brought a group of workers' children to see the privileged lifestyle led by the rich. Maria and the children are quickly ushered away, but Freder is fascinated by Maria and descends to the workers' city in an attempt to find her.
Freder finds himself in the machine rooms and watches in horror as a huge machine explodes, causing several injuries and deaths, after one of its operators collapses from exhaustion. Freder runs to tell his father. Grot (Heinrich George), one of the foremen, arrives soon afterward to deliver maps found on the bodies of the dead workers. Fredersen is angered that his assistant Josaphat has failed to be the first to bring him news of either the explosion or the maps, and fires him. Knowing that he can only go into the depths and become a worker, Josaphat attempts suicide but is stopped by Freder, who sends him home to wait for him. Concerned by Freder's unusual behavior, Fredersen dispatches the Thin Man to keep track of his movements.
Returning to the machine rooms, Freder encounters the worker Georgy and takes his place when he collapses at his post. The two men trade clothes, with Freder instructing Georgy to go to Josaphat's apartment and wait for him. However, while being driven away by Freder's chauffeur, Georgy becomes distracted by the sights and sounds in the licentious Yoshiwara nightclub and spends the evening there instead. Meanwhile, Freder finds a map in his pocket and learns of a secret meeting from another worker as he suffers hallucinations brought on by the exhausting shift.
Fredersen takes the maps brought by Grot to the inventor Rotwang in order to learn their meaning. Rotwang had been in love with a woman named Hel, who left him to marry Fredersen; she died giving birth to Freder. He has since built a robot (a Maschinenmensch, or Machine-Human) to "resurrect" her. The maps show the layout of a network of ancient catacombs beneath Metropolis, and the two men leave to investigate. They eavesdrop on a gathering of workers, including Freder, and find Maria waiting to address them.
Maria prophesies the arrival of a mediator who can bring the working and ruling classes together, and urges the workers to have patience. Freder comes to believe that he could fill the role, and after the meeting breaks up, he declares his love for her. They agree to meet in the city cathedral the next day, then part. Fredersen orders Rotwang to give Maria's likeness to the robot so that it can ruin her reputation among the workers, but does not know of Rotwang's secret plan to destroy Freder as revenge for losing Hel. Rotwang chases Maria up through the catacombs and kidnaps her.
The next morning, the Thin Man catches Georgy leaving Yoshiwara, orders him to return to his post, and takes Josaphat's address from him. Freder goes to Josaphat's apartment in search of Georgy, but finds that Georgy never arrived. After telling Josaphat of his time in the workers' city, Freder leaves for the cathedral, just missing the arrival of the Thin Man. Josaphat rebuffs the Thin Man's attempts to bribe and intimidate him into leaving Metropolis; the two fight, and Josaphat escapes to hide in the workers' city.
Freder does not find Maria at the cathedral, but he does overhear a monk preaching about the Whore of Babylon and an approaching apocalypse. Coming across statues of Death and The Seven Deadly Sins, he begs them not to harm Maria, then leaves to search for her. He hears her cries while passing Rotwang's house and ends up trapped inside until the robot has been fully transformed into Maria's double. Rotwang sends her to greet Fredersen; Freder finds the two embracing in his office and faints, falling into a prolonged delirium. The false Maria begins to unleash chaos throughout Metropolis, driving men to murder out of lust for her in Yoshiwara and stirring dissent amongst the workers.
Freder recovers 10 days later and seeks out Josaphat, who tells him of the spreading trouble. At the same time, the real Maria escapes from Rotwang's house after Fredersen breaks in to fight with him, having learned of Rotwang's treachery. Descending to the catacombs, Freder and Josaphat find the false Maria urging the workers to rise up and destroy the machines. When Freder accuses her of not being the real Maria, the workers recognize him as Fredersen's son and rush him, but Georgy protects him and is stabbed to death. Fredersen orders that the workers be allowed to rampage, so that he can justifiably use force against them at a later time.
The workers follow the false Maria from their city to the machine rooms, unknowingly leaving their children behind. They abandon their posts and destroy the Heart Machine, the central power station for Metropolis, after its foreman Grot reluctantly grants them access to it on Fredersen's orders. As all systems above and below ground fail, Maria descends to the workers' city, which begins to flood due to the stopped water pumps. She gathers the children in the main square, and with help from Freder and Josaphat, they escape from the workers' city as it crumbles in the flood.
In the machine rooms, Grot gets the attention of the wildly celebrating workers and berates them for their out-of-control actions. Realizing that they left their children behind in the now-flooded city, the workers go mad with grief and storm out to avenge themselves upon the "witch" (the false Maria), who spurred them on and has since slipped away to join the revelry at Yoshiwara. Meanwhile, Rotwang has fallen under the delusion that Maria is Hel and sets out to find her. The mob captures the false Maria and burns her at the stake; a horrified Freder watches, not understanding the deception until the outer covering disintegrates to reveal the robot underneath.
Rotwang chases Maria to the roof of the cathedral, pursued by Freder, and the two men fight as Fredersen and the workers watch from the street. Josaphat tells the workers of their children's safety to stop them from harming Fredersen. Rotwang loses his balance and falls to his death. On the cathedral steps, Freder fulfills his role as mediator ("heart"), linking the hands of Fredersen (the city's "head") and Grot (its "hands") to bring them together.