June 1946. As the residents of Winthrop Island prepare for the first summer season after the sacrifice of war, a glamorous new figure moves into the guest cottage at Summerly, the idyllic seaside estate of the wealthy Peabody family. To Emilia Winthrop, daughter of Summerly’s year-round caretaker, Olive Rainsford opens a window into a world of shining possibility. The heady promise of Peabody patronage is blown apart by the arrival of Sumner Fox, an FBI agent who demands Emilia’s help to capture a Soviet agent who’s transmitting vital intelligence on the West’s atomic weapon program from somewhere inside the Summerly estate. Eight years later, shocking news arrives from Washington—the traitor Emilia helped convict is about to be swapped for an American spy imprisoned in the Soviet Union, but with a mysterious condition only Emilia can fulfill.