A married couple is traveling on a train from Boston to New York City. They meet a mysterious silent man known only as Mister If, who shows them a small portable television-like device about 6 by 9 inches (15 cm × 23 cm) in size. (Mr. If's first name is implied to be What, but whether this is true or not is left to the reader.) On the device's screen, Mr. If shows the couple scenes from their earlier lives and what might have happened if certain minor but pivotal events had not occurred.