Modern Times portrays Chaplin as a factory worker employed on an assembly line. There, he is subjected to such indignities as being force-fed by a "modern" 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, 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 gets out of the jail. When he returns, he stumbles upon a jailbreak and knocks the convicts unconscious. He is hailed a hero and is released.
Outside the jail, he applies for a new job but leaves after causing an accident. He runs into an orphaned girl, the Gamin (Paulette Goddard), who is fleeing the police after stealing a loaf of bread. 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 the Gamin in the paddy wagon, which crashes, and the girl convinces him to escape with her. Dreaming of a better life, he gets a job as a night watchman at a department store, sneaks the Gamin into the store, and even lets burglars have some food. Waking up the next morning in a pile of clothes, he is arrested once more.
Ten days later, the Gamin 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. He gets his boss trapped in machinery, but manages to extricate him. The other workers decide to go on strike. Accidentally paddling a brick into a policeman, he is arrested again. Two weeks later, he is released and learns that the Gamin is a café dancer. She tries to get him a job as a singer and a waiter. At his new job, however, he finds it difficult to tell the difference between the "in" and "out" doors to the kitchen, or to successfully deliver a roast duck to table through a busy dance floor. During his floor show, he loses a cuff that bears the lyrics of his song, but he rescues his act by improvising the story using an amalgam of word play, words in (or made up of word parts from) multiple languages and mock sentence structure while pantomiming. His act proves a hit. When police arrive to arrest the Gamin for her earlier escape, they escape again. The Gamin despairs that there's no point to their struggling, but the factory worker assures her that they'll make it somehow. In the final scene, they walk down a road at dawn, towards an uncertain but hopeful future.