In 1939, barred from places that once felt like home and stripped of their possessions, Hannah Rosenthal and her best friend Leo Martin make a pact: Come what may, they promise to have a future together. A chance to have that future appears in the form of the St. Louis, a transatlantic liner that can supposedly provide Jews safe passage to Cuba. Seven decades later, Anna Rosen receives a package from Hannah, a great aunt she has never met. She and her mother travel to Havana to meet her, and learn the story of how Hannah and Leo honored the solemn pact they made.