Modern Times portrays Chaplin in his Tramp persona as a factory worker employed on an assembly line. There, he is subjected to such indignities as being force-fed by a malfunctioning "feeding machine" and an accelerating assembly line where he screws nuts at an ever-increasing rate onto pieces of machinery. He finally suffers a nervous breakdown and runs amok; getting stuck within a machine and throwing the factory into chaos. He is sent to a hospital. Following his recovery, the now unemployed factory worker is mistakenly arrested as an instigator in a Communist demonstration. In jail, he accidentally ingests smuggled cocaine, mistaking it for salt. In his subsequent delirium, he avoids being put back in his cell. When he returns, he stumbles upon a jailbreak and knocks the convicts unconscious. He is hailed as a hero and given special treatment. When he is informed that he will soon be released due to his heroic actions, he argues unsuccessfully that he prefers life in jail.

Outside of jail, he applies for a new job but leaves after causing an accident. He runs into a recently orphaned barefoot girl, Ellen (Paulette Goddard), who is fleeing the police after stealing a loaf of bread. Determined to go back to jail and to save the girl, he tells police that he is the thief and ought to be arrested. A witness reveals his deception and he is freed. To get arrested again, he eats an enormous amount of food at a cafeteria without paying. He meets up with Ellen in a paddy wagon, which crashes, and she convinces him to escape with her. Dreaming of a better life, he gets a job as a night watchman at a department store, sneaks Ellen into the store, and encounters three burglars: one of whom is "Big Bill", a fellow worker from the factory at the beginning of the film, who explains that they are hungry and desperate. After sharing drinks with them, he wakes up the next morning during opening hours and is arrested one more time for failing to call the police on the burglars and for sleeping in store's clothes on a desk thus shocking a customer and the storekeeper.

Ten days later, Ellen takes him to a new home – a run-down shack that she admits "isn't Buckingham Palace" but will do. The next morning, the factory worker reads about an old factory re-opening and lands a job there as a mechanic's assistant. His boss accidentally falls into the machinery, but the worker manages to extricate him. The other workers suddenly decide to go on strike. Outside, the worker accidentally launches a brick at a policeman and is arrested again.

Two weeks later, he is released and learns that Ellen is a café dancer. She gets him a job as a singer and waiter, where he goes about his duties rather clumsily. During his floor show, he loses his cuffs, which bear the lyrics to his song, but he rescues the act by improvising the lyrics using gibberish from multiple languages, plus some pantomiming. His act proves a hit. When police arrive to arrest Ellen for her earlier escape, the two flee again. Ellen despairs that there's no point to their struggling, but the factory worker assures her that they'll make it somehow. At a bright dawn, they walk down the road towards an uncertain but hopeful future.