In the future, wealthy industrialists and business magnates and their top employees reign over the city of Metropolis from colossal skyscrapers, while underground-dwelling workers toil to operate the great machines that power it. Joh Fredersen is the city's master. His son, Freder, idles away his time at sports and in a pleasure garden, but is interrupted by the arrival of a young woman named Maria, who has brought a group of workers' children to witness the lifestyle of their rich "brothers". Maria and the children are ushered away, but Freder becomes fascinated by her and goes to the lower levels to find her. In the machine halls, he witnesses the explosion of a huge machine that kills and injures numerous workers. Freder has a hallucination that the machine is a temple of Moloch and the workers are being fed to it. When the hallucination ends, and he sees the dead workers being carried away on stretchers, he hurries to tell his father about the accident.

Grot, foreman of the Heart Machine, brings Fredersen secret maps found on the dead workers. Fredersen fires his assistant Josaphat for not being the first to bring him details about the explosion or the maps. After seeing his father's cold indifference towards the harsh conditions faced by the workers, Freder secretly rebels against him by deciding to help the workers. He enlists Josaphat's assistance and returns to the machine halls, where he trades places with a worker who has collapsed from exhaustion.

Fredersen takes the maps to the inventor Rotwang to learn their meaning. Rotwang had been in love with a woman named Hel, who left him to marry Fredersen and later died giving birth to Freder. Rotwang shows Fredersen a robot he has built to "resurrect" Hel. The maps show a network of catacombs beneath Metropolis, and the two men go to investigate. They eavesdrop on a gathering of workers, including Freder. Maria addresses them, prophesying the arrival of a mediator who can bring the working and ruling classes together. Freder believes he can fill the role and declares his love for Maria.

Fredersen orders Rotwang to give Maria's likeness to the robot so that it can discredit her among the workers, but is unaware that Rotwang plans to use the robot to destroy Metropolis and ruin both Fredersen and Freder. Rotwang kidnaps Maria, transfers her likeness to the robot, then sends the robot to Fredersen. Freder finds the two embracing and, believing it is the real Maria, falls into a prolonged delirium. Intercut with his hallucinations, the false Maria unleashes chaos throughout Metropolis, driving men to murder and stirring dissent among the workers.

Freder recovers and returns to the catacombs, accompanied by Josaphat. Finding the false Maria urging the workers to rise up and destroy the machines, he accuses her of not being the real Maria. The workers follow the false Maria from their city to the machine halls, leaving their children behind. They destroy the machines, triggering a flood in their city deeper underground. The real Maria, having escaped from Rotwang's house, rescues the children with help from Freder and Josaphat. Grot berates the celebrating workers for abandoning their children in the flooded city. Believing their children to be dead, the hysterical workers capture the false Maria and burn her at the stake. A horrified Freder watches, not understanding the deception, until the fire reveals her to be a robot. Rotwang becomes delusional, seeing the real Maria as his lost Hel, and chases her to the roof of the cathedral, pursued by Freder. The two men fight as Fredersen and the workers watch from the street, and Rotwang falls to his death. Freder fulfills his role as mediator by linking the hands of Fredersen and Grot to bring them together.