During a Mardi Gras parade in New Orleans, a girl becomes the latest victim of the deadly virus "XB". Dr. Clinton Earnshaw has been following the outbreak but only is able to diagnose it. The federal government assigns him Jeff Adams, who has no medical or scientific training. Though Earnshaw is initially bemused by the assignment, Adams' value emerges when he remembers the 19th century discovery of a virus with similar characteristics. Known at the time as "Wood's Fever", it was discovered by Dr. Joshua P. Henderson. Both men know that Henderson's notes were destroyed in the 1871 Great Chicago Fire, his only remaining artifact a gold pocket watch. Adams introduces Earnshaw to a former NASA physicist and Nobel laureate, Dr. Amos Cummings, and his colleague Dr. Helen Sanders. The physicists have been experimenting with time travel and reveal their plan to send Earnshaw and Adams back in time to find Henderson's cure for Wood's Fever. After being outfitted with period gear, clothing, a small microscope and portable centrifuge, Earnshaw and Adams are briefed on the dangers of time travel. They step through a vault-like door into a room with a view of endless cloud-filled sky, and the process begins. On arrival, they discover that the machine has placed them a day and a half later than expected in the center of Chicago, instead of near Henderson's home outside the city with three days to achieve their goal. Posing as doctors sent from Washington, DC, they find Henderson at his hospital and ask him about his success with the virus. Henderson confesses that he does not know why his patients are surviving. His treatment involves palliative drugs washed down with homemade elderberry wine. Adams and Earnshaw perform their own tests on Henderson's patients, but are equally baffled by their recovery. While trying to obtain a blood sample from one of the survivors, Adams initially thinks he's killed the man but discovers that he had not been cured and would have died anyway. Their mission is further complicated when Earnshaw and Henderson's niece, Jane, fall in love. Earnshaw contracts the virus while working with Henderson's patients. When the Great Fire breaks out, the travelers are no closer to the cure. Adams decides to take all of Henderson's paperwork and return to the future, but he is interrupted when a suspicious assistant enters the room. The assistant holds a telegram from Washington, responding to Henderson's cable of thanks with the information that Adams and Earnshaw were not sent by the government. Earnshaw realizes a substance in Henderson's homemade wine is the cure; the only patients who succumbed were those who refused to take alcohol. The race is on to recover the last bottle of wine. They show Henderson the futuristic microscope and centrifuge and recite the inscription inside his pocket watch, convincing him that they are from the future. Despite knowing their fate, the Hendersons remain at the hospital with a trapped patient and perish in an explosion. The travelers return to the departure point with the wine just in time. In 1976, Henderson's wine saves Earnshaw and also the girl stricken during Mardi Gras. After the success of their mission, Adams and Earnshaw visit the graves of Henderson and his niece in Chicago. They place a letter of thanks from the President on Henderson's grave. Earnshaw admits that in loving Jane he "fell in love with history".
