The story of King Casimir III the Great and his second wife Queen Adelaide of Hesse, is extended of threads of his sister - Elizabeth of Poland, Queen of Hungary, her son King Louis the Great and his nephew - Duke Bolko II the Small, his wife Agnes von Habsburg and niece Anna of Świdnica, future empress.
Season shows also bigamic marriages of Casimir with Christina Rokiczana and Hedwig of Sagan, during Queen Adelaide's life.
The action of the final episode takes place in 1374, four years after Casimir's death, when Jadwiga of Poland is born.
One of the main threads is her relationship with Prince William Courteous of Austria and marriage with the Lithuanian Grand Duke Jogaiła.
The fourth season covers 1399–1420, it begins two months after the death of Queen Jadwiga, when the bereaved Jogaila considers returning to Lithuania.
The season also portrays Jogaila's relations with his cousin Vytautas, Grand Duke of Lithuania and rebellious brother Svitrigaila, as well as the growing conflict with the Teutonic Order, leading to the Battle of Grunwald in 1410 and the King's love marriage with widowed Elizabeth Granowska with her death marking the season finale.