This novel of speculative fiction tells the story of a lonely, frustrated artist in New York City who journeys back and forth through time. It is in many ways written in the tradition of similar novels written by H.G. Wells and Jules Verne, using real-life science as a springboard for a narrative that explores the possibilities of what might happen if the principles behind that science were taken further. Within that context, the novel explores themes of personal responsibility and the relationship between past and present.

The novel begins as Simon (Si) Morley's regular day at work as a graphic artist in advertising is interrupted by the arrival of an unexpected visitor, Ruben Prien, who invites him to participate in a mysterious government project. At first Si is hesitant, but is soon convinced to give the project a try. He then undergoes a bewildering series of tests, the results of which prove he's an exceptionally good candidate - but for what, exactly, is still unclear.

Shortly after the tests are completed, Si is introduced to Danziger, the head of the project. Danziger explains that he and other scientists are exploring the theory that it's possible that the past exists at the same time as the present, and that a person can travel into it. He also tells Si he has more potential for successfully completing such a journey than any of the other potential candidates, and tells him what period in the past the scientists want him to visit. Si has other ideas, and explains his interest in visiting a moment at which a mysterious event in the family history of his girlfriend Katie occurred. After debating Si's proposal with board members overseeing the project, Danziger agrees. Si then begins an extensive process of training in which he's taught about the manners, history, and attitudes of the period of the past to which he's been assigned - the winter of 1882 in New York City.

The training process is soon complete, and Si makes a very brief journey into the past. Once Danziger and the other scientists are satisfied that the present hasn't changed in any substantial way, they agree that Si should go back for a longer visit and investigate the incident. Si makes the journey and observes the incident, tracking one of the participants to a particular boarding house and then returning to the present. Once more the scientists are satisfied the present hasn't changed, and plans are made for Si to go back into the past once again, on an even longer visit to conduct further investigations.

Si once again completes the journey into the past, taking a room at the boarding house. While there, he falls in love with Julia Charbonneau, a young woman running the place. Conflict arises when Si discovers that Julia is involved with the emotionally volatile Jake Pickering, who also happens to be the suspicious character Si followed to the boarding house. Si's detective work continues at the same time as his feelings for Julia deepen, bringing him into a tense relationship with Jake. This eventually leads Si to tell Julia what he's learned about Jake, and she insists upon accompanying him when he goes out to observe a mysterious rendezvous between Jake and another man, Carmody, who is the mysterious family member in Katie's past.

Neither Jake's plans nor Si's plans proceed as they'd anticipated. They both, along with Julia and Carmody, are trapped in a deliberately set and quickly spreading fire. Si and Julia escape, both feeling guilt at having been at least partially responsible for the fire but both realizing there was nothing that could be done. A visit by the ruthless Chief of Police convinces them that there is a plot to make them criminally responsible, and they spend two very tense days on the run, ending only when Si makes the desperate choice to take Julia back to the present with him.

Si spends a pleasant day educating the sometimes wonderstruck, sometimes fearful Julia about the 20th Century. At the end of the day, however, she announces her decision to return. When Si expresses concern that the police will still be after her, Julia reveals that she knows the truth of what happened in the fire, which leads Si to understand the truth behind the mysterious incident involving Carmody. Julia returns to the past and Si returns to the scientists at the project, explaining what happened and expressing his desire to leave the project. The scientists tell him they want him to go back one more time and change a crucial event in the past to improve the present. Si refuses, saying no one has the right to tamper with reality that way. They insist, he reconsiders, and arrangements are made for him to return. Conversation with Danziger, however, convinces him that his original impulse was right - the past should remain as it is and the plans to change it can't be allowed to develop further. Si agrees, and when he returns to the past, he eliminates the possibility that the entire project could be started in the first place. He then returns to the boarding house, preparing to start a new life with Julia.