Peter Corrigan is attending the Potomac Club in Washington D.C. and discussing the theoretical concept of time travel with his fellow card players. Corrigan argues that history can't be changed and then bids the players goodnight. As he goes he bumps into a tray carried by William, the club's attendant. Corrigan checks the date, April 14, 1961, and then walks outside. As he steps through the door, he's suddenly staggered... and finds himself transported back to April 14, 1865, wearing clothing of the era. Confused, Corrigan heads for his home only to find that it's a boarding house. he gets a room and meets a couple who are going to Our American Cousin at Ford's Theater... on April 14, 1865. Corrigan realizes that Lincoln will be assassinated that night and storms out.
Corrigan goes to Ford's Theater and pounds on the door trying to get in. The police arrest him and take him to the station where they dismiss his claim after he can't account for his claim. They assume he's drunk and take him to a cell. A gentleman, Jonathan Wellington, notes the situation and offers to take Corrigan under his care assuming he's a war veteran suffering from shell shock. One officer wonders if they should investigate Corrigan's claim but the sergeant tells him they have no basis to intervene.
Wellington takes Corrigan back to the former's rented room and pours him a drink, then says he's a psychiatrist with the government and is interested in knowing how Corrigan learned of the assassination. He's interested in helping Corrigan if he can prove his claim, and offers him a handkerchief for his head injury. Corrigan collapses and realizes that he's drugged. Wellington assures him that he will return later but Corrigan needs his rest and departs.
As the play begins, the barely-conscious Corrigan tries to get to the door and as he calls for help, collapses. A few minutes later, the landlady lets the officer from the station into the apartment. The officer has been trying to get the authorities to add more protection to the President's security force. The landlady has no such boarder named Wellington, and Corrigan finds that the handkerchief Wellington loaned him has the initials "JWB" on it. Realizing he's been tricked by John Wilkes Booth, they hear people outside talking about the death of the President. As the others leave, Corrigan pounds on the wall and complains that no one listened...
... and finds himself back at the door of the Potomac Club in 1961. A different attendant greets him, a man Corrigan has never seen before. The man knows Corrigan and says he stepped out just a few seconds ago. He goes back to his fellow card players and realizes there's no way to explain what happened. They're talking about making money and there's a new man at the table: William. William's great grandfather was the officer who believed Corrigan, and with the public acclaim from his efforts to stop the assassination, rose to become chief of police and then finally a millionnaire. William has no idea why Corrigan thinks he was an attendant. Corrigan notes that in the matter of time travel, things can be changed. They wonder why he's acting strangely, while Corrigan discovers that he has a handkerchief in his pocket... with the initials "JWB" on it.