Ageing author A.V. Roe boards the Amtrak Empire Builder in La Crosse, Wi on his way home to Oregon during his summer vacation. He is putting the finishing touches on his last novella. Coincidentally, a succubus named Cynthia is taking a vacation break between assignments, having decided to take the train for a leisurely break in her duties. Traveling from Chicago to Washington state to meet her next victim, they end up in adjoining sleeper compartments and she senses an uncanny attraction to the writer of erotic sexual fantasy. During the first night of the trip, she enters his compartment and gives him an unforgettable wet dream. While in the compartment, she sees the book he is editing on his computer and reads his last story, only to realize that she is his protagonist. Curious about all of his stories, she reads the other works he has written that are stored on his computer and becomes familiar with his polyamorous characters.
The next day, having taken the form of his wet dream heroine, a real woman from his past, they bond in a tumultuous sexual encounter after he accepts the inevitable if they become lovers. The rest of the trip is filled with his ultimate sexual fantasies as he spends a day with each of his main characters in the sexual escapades his stories are noted for. However, for Cynthia, something entirely unexpected befalls her. The possibility she is falling in love with a mortal, something that is beyond her comprehension.
Their trip reaches its ultimate climax at the author's Last River To Cross.