On January 5, 1900, four friends arrive for a dinner at a house located near London, but their host, H. George Wells, is absent. As requested, they begin without him, but then George staggers in, exhausted and disheveled. He begins to recount his adventures since they last met on New Year's Eve, 1899. A week earlier, George discusses time as "the fourth dimension" with friends, among them David Filby (Young) and Dr. Philip Hillyer (Sebastian Cabot). He shows them a tiny machine that he claims can travel in time, stating that a larger version can carry a man "into the past or the future". When activated, the device blurs and disappears. Most of his friends dismiss it as a trick, but after the others have gone, Filby warns George to destroy the machine. They agree to meet again next Friday with the others. George uses the Time Machine to travel to the future. He first leaves the machine on September 13, 1917, where he meets James Filby (Young again), whom he mistakes for his father, David. James informs George that his father had "died in the war", and that the United Kingdom has been at war with Germany since 1914. He tells him that an inventor lived across the road who disappeared around the turn of the century. George then travels to June 19, 1940, into the midst of "a new war", which he briefly stops in as his machine is buffeted from side to side. George's next stop is August 19, 1966, in a futuristic metropolis. He is puzzled to see people hurrying into a fallout shelter amid the blare of air raid sirens. An older James Filby tells him to get into the shelter. James spots an atomic satellite zeroing in and flees into the shelter. A nuclear explosion causes a volcano to erupt. Civilization is destroyed in a nuclear holocaust. George restarts the machine just in time to avoid being incinerated, but lava covers the machine, then cools and hardens, forcing him to travel far into the future until it erodes away. He stops the machine on October 12, 802,701, next to a low building with a large sphinx on top. George explores, and spots young people by a river. A woman is drowning, but the others are indifferent. George rescues her, but is surprised by her lack of gratitude or other emotion. She calls herself Weena (Mimieux) and her people the Eloi. Outraged by the Elois' apathy and lack of ambition, George returns to where he had left his time machine, to find that it has been dragged into the sphinx-building, behind locked metal doors. Weena follows George and insists they go back, for fear of "Morlocks" at night. A monster jumps out of the bushes and tries to drag Weena off, but George rescues her and wards the beast off with fire. Weena informs him that the hideous creature was one of the Morlocks. The next day, Weena shows George what appear to be domed well-like air-shafts in the ground. She then takes him to an ancient museum, where "talking rings" tell of a centuries-long nuclear war/holocaust. One group of survivors remained underground in the shelters and evolved into the Morlocks, while the other group, which became the Eloi, returned to the surface. George starts climbing down a shaft, but turns back when a siren begins blaring from atop the sphinx-building. Weena and the rest of the Eloi enter a trance-like state, and complacently file through the now-open doors of the building. When the siren stops, the doors close, trapping Weena and others inside.
To rescue Weena, George climbs down a shaft and enters the subterranean caverns and is horrified to discover that the Eloi are little more than freerange livestock to the Morlocks, who raise and cannibalise them. He fights the Morlocks with the help of the Eloi, who prove to be not completely helpless, then escapes with them up the shafts to safety. Under his direction, they drop dry dead tree branches into the shafts to feed the fire. The entire area caves in, crushing and suffocating most of the Morlocks below. The next morning, George finds the sphinx-building in charred ruins and the doors to the building open again, with his time machine sitting just inside the entrance. He goes to retrieve his machine, but the doors close behind him and he is attacked by the remaining Morlocks. He uses the time machine to escape back to January 5, 1900. George's friends scoff at his story and leave; only Filby believes him. George leaves again in the time machine.