American atomic scientist Dr. Peter Standish toils so relentlessly at a nuclear laboratory in London that his supervisor, Dr. Ronson, and co-worker, Roger Forsyth, worry that Standish's intense dedication to his work will ruin his health. At the end of one day, Forsyth gives Standish a ride to his eighteenth-century residence in Berkeley Square and suggests that he take a vacation. Standish, who inherited the house from a distant ancestor, explains to Forsyth that he will be leaving for a while. Forsyth's concern for Standish's mental health deepens as Standish explains that the house was once owned by the first Peter Standish, an American who came to London to marry his cousin, Kate Pettigrew, in 1784. Standish has found a diary and other documents belonging to his ancestor and, due to their strange nature, believes that he will be able to switch places in time with the first Standish that very night. Forsyth states that Standish's obsession with the eighteenth century is crazy, but Standish maintains that it was a time of reason and graciousness, much preferable to the chaos of the current century. Forsyth eventually leaves, and after Standish sees him to his car, a bolt of lightning hits him as he re-enters the house. When Standish regains consciousness, he discovers that he is wearing different clothes, and upon entering the house, he is greeted by Kate Pettigrew; her mother, Lady Anne; her brother Tom and their friend, Mr. Throstle. After realizing that he has gone back in time, Standish recovers his composure and delights in meeting the people about whom he already knows so much. Standish is taken aback, however, by the arrival of Kate's lovely sister Helen, about whom there was no mention in the historical papers. As the days pass, the Pettigrews are disturbed by Standish's eerie foreknowledge of events and his odd manners, although Helen is attracted to him. Standish returns Helen's feelings, but becomes disenchanted with the eighteenth century when he discovers that the majority of the population live in filth and poverty. The Pettigrews and their social circle are more narrow-minded than Standish had imagined, and he loses patience with their prejudices. Upon meeting renowned painter Joshua Reynolds, Standish reveals his knowledge of Reynolds' painting entitled "The Tragic Muse," even though Reynolds has just begun the project and not yet told anyone its name. Such occurrences prompt Kate to fear her cousin, and she also grows nervous about the attention he pays to Helen. Standish confides in Helen that he is from the future and takes her to his secret laboratory, where he has been "inventing" photography, a steamship, electricity and other innovations that he hopes will help the common people. Helen is not shocked by Standish's revelations, and drawing comfort from her support, he calls her "the only real beauty in this ugly century." That night, at a party hosted by the Pettigrews, Standish ignores Kate to dance with Helen, much to the chagrin of Throstle, who hopes to marry Helen himself. Standish meets the famed Duchess of Devonshire, Dr. Johnson and James Boswell, but when he praises the duchess, she is chilled by his statements, for he sounds as if he were reading her obituary. Having thus frightened more of his acquaintences, Standish confronts Kate, who declares that she will not marry him. Standish insists that she will, and that they will have two children, further upsetting her. Kate storms out, and after the party, Helen questions Standish about the future. When she gazes into his eyes, Helen sees scenes of Standish's life in the future, but is not frightened. Standish and Kate profess their love, and Kate assures him that their love will exist for all time. Soon after, Throstle notifies the Bow Street magistrate about Standish's laboratory, and after his experiments are destroyed by the fearful constables, Standish is questioned by Sir William Sutherland, a physician. Despite Standish's pleas as one scientific man to another, Sutherland decides to commit him to Bedlam. Standish rushes to Helen's room to say goodbye, and she places a crux ansata , an Egyptian symbol of life and eternity, in a hidden drawer in her desk to remind him of her love for him. Helen swears her eternal love for Standish, and he is then escorted out by the guards. Lightning strikes him again, however, and Standish regains consciouness back in his own time. Forsyth arrives and informs him that he has been having a nervous breakdown for the past seven weeks. When Forsyth describes his unusual behavior, Standish realizes that he did change places with his ancestor, who was horrified by modern life. Standish then goes to Helen's old room and finds the crux ansata , after which Forsyth is joined by his sister Martha, who had helped to care for Standish during his illness. Standish is amazed at Martha's appearance, as she is an exact double for Helen. Standish then goes to the churchyard where Helen had said that she would be buried, and there he learns that she died soon after he and the other Standish switched places again. Remembering Helen's promise that they would be together in God's time, Standish gazes at Martha and leaves with her.