Salt to the Sea takes places in East Prussia in January 1945. Thousands of refugees try to evacuate as World War II draws to a chaotic close.
Four young narrators from different nations experience the terrors of war. Emilia, a fifteen-year-old Polish orphan, follows Florian, an East Prussian restoration artist. She views him as her "knight" after he saves her from a Russian soldier. Both run into Joana, a Lithuanian nurse who is traveling with a group of refugees. All are trying to make their way to West Germany, to the ports of Gotenhafen and Pillau, in the hopes of boarding evacuation ships to safety. Meanwhile, Alfred, a bumbling and hateful Nazi sailor, is stationed at the port of Gotenhafen. There he prepares a ship called the MV Wilhelm Gustloff for the evacuation.
As the refugees undertake the long and difficult journey, they gradually reveal their secrets. Emilia is eight months pregnant after Russian soldiers raped her. She is also persecuted for being Polish and must hide her identity. Florian is also on the run because he stole Hitler’s favorite piece of art from the Amber Room. He did this to take revenge when he realized the Nazis were using his talent to help steal Europe's artistic treasures. Joana feels responsible for abandoning her family and leading to some of their deaths.
By the time the group of refugees finally arrives to the port of Gotenhafen, their lives are increasingly intertwined. Joana and Florian fall in love. Emilia views Florian as proof that there are good men in the world. At Gotenhafen, they all depend on Alfred’s help to get boarding passes.
On board the Gustloff, Joana works in the infirmary while Florian hides from the Nazi soldiers who are looking for him. Emilia gives birth to her baby, Halinka. Suddenly, Russian torpedoes hit the Gustloff. Within an hour the ship sinks and thousands die. Joana and Florian escape on a lifeboat with Halinka and Klaus, a young boy who traveled with their group. Emilia ends up on a raft with Alfred. Emilia reveals she is Polish and Alfred tries to kill her. The reader learns that despite Alfred's racist ideology, his love interest, Hannelore, is Jewish. In the end both Alfred and Emilia die.
Years later, Joana and Florian live with Halinka, Klaus and a child of their own in the United States. A Danish woman named Clara Christensen sends a letter to Florian, telling her that Emilia’s body washed up on her shore, and that she has buried the body and the items in Florian's pack.
