A mysterious and shocking incident takes place one night in Venice. The beautiful Marchesa di Mentoni drops her only infant from the Ducal Palace window into the Grand Canal. Yet her attention seems distracted, while the rescuers search the water for the drowning child, and her gaze is fixed on the building of the Old Prison opposite.
Suddenly a stranger steps out of the shadows, plunges into the dark waters, and appears with the baby, who miraculously is still alive. The child is whisked away from its mother, and a brief and mysterious exchange between the Marchesa and the stranger sheds no light for the reader on the situation.
The narrator, who has observed these events from his gondola now offers a lift home to the stranger, and visits him on request at dawn the next morning. Here an unprecedented series of events occurs which is both astounding and terrible, but from which the listener is able to piece together what may have been behind the events of the preceding evening.