On January 5, 1900, four friends arrive for a dinner at the London home of their inventor friend George. He is absent, then suddenly appears, bedraggled and exhausted. He recounts what happened to him.
At the group's earlier dinner on New Year's Eve, George stated that time is the fourth dimension. He shows David Filby, Dr. Philip Hillyer, Anthony Bridewell, and Walter Kemp a scale model time machine. When a tiny lever on it is pressed, the device quickly disappears. George says it went forward in time, but his friends are skeptical. The group leaves George's house, Filby reluctantly, as he senses George is not himself. George then retires to his private laboratory that holds a full-size time machine.
George travels forward in time, first in small increments, and then to 1917. He meets Filby's son, James, who says Filby died in a war. George returns to the time machine and stops in 1940 during the Blitz, finding himself in the midst of "a new war". A disillusioned George then travels to 1966. People are rushing to fallout shelters as air raid sirens are blaring. An elderly James Filby urges George to take cover. George barely makes it back to his time machine as an "atomic satellite" detonates, causing a local volcanic eruption. The approaching lava rises, cools, and hardens, trapping George as he travels far into the future. Eventually the lava wears away, revealing a lush, unspoiled landscape.
George stops on October 12, 802,701, near the base of a sphinx. He encounters young men and women wearing simple clothing gathered by a stream. One woman, carried off by the current, screams for help. When her indifferent companions do nothing, George rescues her. She is named Weena and her people are the Eloi; they do not operate machines, work, or read, and know little of their history. Their food is always provided for them. One young man shows George a library, but the books crumble to dust when touched. Outraged, he decides to leave, but his machine has been dragged inside the closed sphinx. Weena, who stays with him, says that Morlocks are responsible, noting they only come out at night. A hideous-looking Morlock jumps out and tries to drag Weena away, but is warded off by George's lit torch.
The next day, Weena shows George domed structures dotting the landscape, air shafts that lead down to the Morlocks' caverns. Weena also shows George an ancient museum where "talking rings" tell of long-ago war between east and west that lasted 326 years and contaminated the atmosphere. Another ring describes humanity's struggle for survival; many people lived underground, while some eventually returned to the surface. George realizes this was the beginning of the speciation that resulted in the Morlocks and Eloi. He starts to climb down a shaft, but stops when sirens emerge and blare from the sphinx. The Eloi go into a trance-like state and head for the opened doors at the sphinx's base. The sirens stop and the doors close, trapping Weena and others inside, while those outside merely walk away.
George enters the caverns through the air shafts. He discovers that the Morlocks raise the Eloi as food. He finds Weena and fights off the creatures, finally inspiring the Eloi to defend themselves. George sets fires and urges the Eloi to climb to the surface. He directs them to drop tree branches down the shafts. The resulting fires cause the caverns to burn, then collapse.
The next morning, the sphinx's doors are open. George sees his time machine inside. When he enters the sphinx, the doors close and George is attacked by Morlocks. He escapes in his machine and returns to 1900.
After George recounts his story, his friends remain skeptical. He produces a flower Weena gave him, and Filby, an amateur botanist, says it is an unknown species. George bids his guests good evening. Filby returns shortly thereafter to find George and his time machine gone. His housekeeper, Mrs. Watchett, notes that nothing is missing except three books that she is unable to identify. When Mrs. Watchett wonders if George will ever return, Filby remarks that "he has all the time in the world".
