In 1893 London, distinguished writer Herbert George "H.G." Wells (Malcolm McDowell) showcases his astonishing invention: a time machine. This revelation unfolds during a dinner party attended by various skeptics. While explaining the intricate workings of the device—particularly the "non-return key," which ensures the traveler remains at their chosen destination, and the "vaporizing equalizer," maintaining balance between the traveler and the machine—the tranquility is shattered when police constables arrive in search of the infamous serial killer, Jack the Ripper. In the ensuing chaos, one officer discovers a blood-stained bag belonging to John Leslie Stevenson (David Warner), a well-regarded surgeon and friend of Herbert, who now goes missing in the home, leading the authorities to suspect him as the notorious murderer.
With urgency, Herbert hurries to his laboratory, only to discover that the time machine has vanished. In a daring move, Stevenson has journeyed to the future, inadvertently leaving behind the "non-return" key, which causes the machine to return to its original time, effectively trapping him in an unknown era. Determined to bring his friend back and confront the dark turn of events, Herbert uses the time machine to pursue Stevenson to November 5, 1979, where he finds the machine unexpectedly displayed in a San Francisco museum. Here, he grapples with the shocking realization that the chaotic reality of the future starkly contrasts his vision of an enlightened socialist utopia. Instead of harmony, he is met with airplanes, automobiles, and a grim history teeming with violence and conflict.
Amidst his search for Stevenson, who might attempt to exchange old currency as an Englishman in America, Herbert encounters Amy Robbins (Mary Steenburgen), a liberated employee at the Chartered Bank of London. She directs him to the Hyatt Regency hotel, where it is revealed that Steven has also visited her in the past. Upon confronting Stevenson, who resides in his hotel room, Herbert learns that his friend has come to enjoy the modern world's violence, admitting with an unsettling sense of pride, > "Ninety years ago, I was a freak. Now... I'm an amateur." The discussion quickly devolves into a struggle over the crucial return key. Disoriented by the arrival of hotel employees, Stevenson escapes, ultimately facing injury in a reckless attempt to flee.
After mistakenly believing Stevenson to be dead, Herbert reconnects with Amy amidst an unexpected romantic tension. She actively pursues him, leading to a budding relationship, while Stevenson plans his next move by trying to extract more cash from the bank. Sensing that Amy had previously guided Herbert to him, Stevenson tracks down her residence, prompting Herbert, in a desperate bid to persuade Amy of the truth of their situation, to leap three days further into the future. To Amy’s horror, they encounter a chilling newspaper headline foretelling her death at the hands of the Ripper, an unnerving reminder of their intertwined fates.
Together, they concoct a plan to return and prevent the grim chain of murders, but their attempts to intervene lead them to an unexpected setback. Trapped in their own fight against time, they can merely alert the authorities, yet it isn't enough to stop Stevenson from claiming another victim. As Herbert becomes privy to Amy's impending danger, he finds himself arrested due to the bizarre circumstances surrounding his knowledge of the unfolding events. Confined and desperate, he perceives the dire threat against Amy while enduring the weight of a twisted fate.
In the aftermath, while Herbert reflects on the horror of losing Amy to Stevenson, he confronts his old friend, who has now kidnapped another innocent woman in order to manipulate Herbert into surrendering the time machine's key. Spiraling into a frantic chase through the streets of San Francisco, Herbert pursues Stevenson with intent, making his way to the museum where the time machine is housed. The tension escalates as they race against time, navigating twisted relationships and dangerous desires.
During the climactic moment, while Stevenson prepares to activate the time machine once more, Herbert capitalizes on a pivotal opportunity to sever the connection between Stevenson and the device. In a decisive moment, he removes the "vaporizing equalizer," forbidding Stevenson from returning to their timeline and effectively banishing him to an endless journey through time—a bold sacrifice to preserve the future while stating, "The time has come to return to my own time, by myself, to destroy a machine that is too dangerous for primitive mankind."
In a heartfelt exchange, Amy begs to accompany him back to the past, despite her initial reservations about Victorian life. As they embark on their journey, she declares her intent to change her name to Susan B. Anthony, symbolizing a new beginning. The end credits portray a poignant image of their future together, revealing that they ultimately married, forever entwined in a tapestry woven by time itself.
