With the third series having left off partway through Winston Graham's sixth Poldark novel The Four Swans, the next run of episodes will draw on the final third of that book and all of the following novel, The Angry Tide.
Demelza finds her loyalties torn Elizabeth tries to strengthen her marriage Morwenna continues to be oppressed by husband Reverend Ossie Whitworth and the Enyses are tested as never before.