Andy Cunningham is on a summer ocean voyage from Charleston, South Carolina, to New York City. He learns that his old friend Cornelius Wyatt is onboard with his wife and two sisters. However, he has reserved three state rooms. After estimating extra room for a servant or extra luggage, he learns that his friend brought on board a rectangular pine box: "It was about six feet long and two and a half wide." Andy notes its peculiar shape and in particular a strange smell coming from it. Nevertheless, he believes that his friend has acquired a particularly valuable copy of Leonardo da Vinci's The Last Supper. Andy has never met Wyatt's wife, but has heard that she is a woman of "surpassed beauty, intelligence and accomplishment".
Andy is surprised to learn that Box shares the state-room with Wyatt and his wife, while the other room is shared by two sisters. For several nights, Andy sees his friend's surprisingly unattractive wife leave the state-room every night at about 11 p.m., go to the third state-room, and return first thing in the morning. While she is gone, Andy believes he hears his friend opening the box and sobbing, which he attributes to "artistic enthusiasm".
As the Independence (the name of the ship) passes Cape Hatteras, it gets caught in a terrible storm. Escape from the damaged ship is made via lifeboat, but Wyatt refuses to part with the box. He issues an emotional plea to Captain Hardy but is denied. Wyatt returns to the ship, and ties himself to the box with rope. "In another moment both the body and the box were in the sea—suddenly, disappeared once and for all."
About a month after the incident, Andy meets with the captain. Hardy explains that the box actually contained the corpse of Wyatt's recently deceased young wife. He had intended to return the body to his mother but bringing a corpse on board would have caused an uproar among the passengers. Captain Hardy had arranged for the box to be registered as baggage only. As both Wyatt and his wife were already registered as passengers, a maid posed as the wife so as not to arouse suspicion.