Salt to the Sea takes place in January 1945, during the final days of WWII. The Allied forces are gaining ground both to the west and the east, and so German civilians are evacuating, fleeing violence and running towards the Baltic Sea where the German navy will transport them to safety.
The story is told from four points of view: Joana is a Lithuanian refugee who was allowed to resettle in Germany; Emilia is a pregnant teenager who escaped the genocide that wiped out many of her fellow ethnic Poles and is trying to remain off the radar of roving German soldiers; Florian is a former art restorer from Prussia who is smuggling a priceless statue he has stolen from the Nazis as revenge after they killed his father; Alfred is a delusional Nazi soldier working on the Wilhelm Gustloff.
Emilia meets Florian in a potato cellar, where he saves her from a Soviet soldier’s attempted assault. Emilia becomes attached to Florian, whom she sees as her “knight,” and begins to follow him on his journey out of East Prussia. That evening, the two hide out in a barn, where they meet Joana and her fellow travelers—Klaus, Eva, Ingrid, and a man called “the Shoe Poet.” Although the other refugees distrust Florian, Joana is a nurse and feels obligated to help him. She removes shrapnel from a wound on his side and stitches the site closed. In the morning, Florian slips out, and Emilia follows him.
The next day, Florian and Emilia accidentally run into Joana again when they all decide to spend the night in the same abandoned Prussian mansion. Although Florian’s wound has begun to heal, Emilia is now clearly sick. Joana examines her and realizes that, although only fifteen, she’s almost nine months pregnant. Joana worries the pregnancy is the result of rape, but Emilia assures her that the father of her child is a man named August, whom she loves and is on her way to meet.
The two groups decide to travel together. They walk for another day until they reach the coast, at which point they must cut across a frozen bay. As they begin to cross the water, Allied planes shoot through the ice and Ingrid, who had been the first to cross, falls into the water and drowns. Shocked by the death of their friend, the group nonetheless makes it across the bay, where they encounter a German soldier. Florian has altered his identification papers to make it look as though he is on a personal mission for Erich Koch, and so the soldier offers to take him by boat wherever he needs to go. Joana and the rest of the refugees convince Florian to let them tag along, and so the group is transported to the city of Götenhafen, a port town where they will be able to board a ship and, they hope, sail to freedom.
In Götenhafen, the group meets Alfred, who is working on the Wilhelm Gustloff. Everyone but Eva manages to get a boarding pass for the same ship—Joana by trading her expertise as a nurse for safe passage, Florian by forging his. Joana begins to work in the maternity ward. There, she cares for Emilia, who gives birth to a baby daughter, Halinka. As she goes into labor, Emilia reveals that Halinka is not August’s child, but the result of rape at the hands of Soviet soldiers. Initially unable to face the prospect of motherhood, Emilia eventually warms to her daughter with the encouragement of her friends and fellow travelers.
Two days after boarding the ship, it finally sets sail. That same day, however, Allied torpedoes pierce the hull and cause the Wilhelm Gustloff to begin to sink. Joana, Klaus, Florian and Halinka make it onto the deck and onto a lifeboat. Emilia asks Florian to carry her baby into the lifeboat. Meanwhile, Florian asks Alfred to briefly hold his pack (which contains a priceless stolen artifact, the amber swan), but then the lifeboat is lowered into the water, and Emilia is separated from her child, and Florian from his belongings.
Joana and Florian are eventually rescued by a boat that comes to save the drowning and freezing refugees, but Emilia and Alfred, although they manage to board a raft, remain adrift at sea. Alfred, who has been writing letters to his beloved Hannelore for much of the novel, reveals that Hannelore was Jewish, and that he turned her into the Nazis when she spurned him. Overcome by rage, delirium, and hypothermia, Alfred first confuses Emilia for Hannelore, and then lashes out when he realizes she is speaking Polish and therefore belongs to a group Hitler has deemed “undesirable.” As he advances on her, Alfred falls into the water and dies. Emilia also freezes to death on the raft, but the final chapter implies that she is reunited with her family and friends in the afterlife.
In a final letter that serves as an afterword, a woman named Clara Christensen writes to Florian about how, twenty years earlier, she found Emilia’s body washed up on the shore by her home. She explains how she buried Emilia, and hopes she is at peace. She adds that she also buried Florian’s backpack, along with the amber swan, and hopes that he is at peace as well.