In 1876, Leopold Alexis Elijah Walker Gareth Thomas Mountbatten, the Duke of Albany and a future elevator inventor, finds himself constrained by his circumstances. His strict uncle, Millard (Paxton Whitehead), is frustrated with Leopold’s irreverence towards the monarchy and presses him to wed a wealthy American, as the Mountbatten family's financial situation has dwindled dangerously. His uncle even told him that "on his thirtieth birthday, he had become a blemish to the family name."
Things take a surprising turn when Leopold encounters Stuart Besser (Liev Schreiber), a budding physicist who seems fascinated by Leopold's schematic drawings. Stuart had previously appeared at a speech by Roebling regarding the Brooklyn Bridge. In an attempt to save Stuart from what he perceives as a suicide, Leopold inadvertently leaps into the same portal that transported Stuart.
Upon awakening in the bustling streets of 21st century New York, Leopold is initially disoriented, fearing he has been abducted. Stuart explains that he has formulated equations to predict temporal portals, advising Leopold to remain hidden in his apartment until the portal reopens the following week. After an unfortunate accident leads Stuart to be hospitalized for discussing his groundbreaking discovery, Leopold meets Kate McKay (Meg Ryan), Stuart's ambitious ex-girlfriend, who enters the apartment looking for her Palm Pilot stylus.
Leopold is captivated by Kate's career-driven lifestyle and remarks on how her profession in market research is commendable for a woman, even reminiscing about a librarian he once dated back in Sussex. However, Kate dismisses him and assigns him the task of walking Stuart's dog. As he adjusts to the modern world, Leopold is awestruck to see that Roebling's bridge still stands. During his stay, he befriends Kate's brother, Charlie (Breckin Meyer), who mistakenly believes Leopold is merely playing a role.
As Leopold and Kate’s relationship blossoms, they explore New York together, sharing meals and experiences. However, when Leopold is cast in a commercial for a product he finds revolting—diet margarine—he struggles with the ethics of endorsing something of questionable value. He passionately argues, > "when someone is involved in something entirely without merit, one withdraws.” In response, Kate insists that sometimes one must do things they dislike, leading to a clash of ideals between the two. She quips, "I don't have time for pious speeches from two hundred-year-old men who have not worked a day in their life," and their romantic connection appears to wane.
Later, Stuart escapes from the mental facility, and as Kate prepares to accept her promotion at a company banquet, he and Charlie rush to find her. Just before her stage debut, they unveil photographs from Stuart's camera that reveal her presence in 1876. Stuart, reflecting on the nature of their situation, contemplates how the timeline is intertwined, stating: "the whole thing is a beautiful 4-D pretzel of kismet inevitability."
Ultimately, Kate chooses a future with Leopold over her career aspirations. The trio makes their way to the Brooklyn Bridge just in time to catch the portal's closing. As Leopold prepares to announce his marriage to Miss Tree, he sees Kate and joyfully declares her name, "Kate McKay," as his bride.
In their final moment, they share a kiss, and the scene pulls back to reveal a wall clock reading 12:15, signaling their timeless love story.
