The Poldarks' peaceful lives are disrupted when an old friend's plea leads them into a world of duplicity.
As George expands his power through a shady alliance, the Poldarks become caught in a deadly web.
Ross's tenacity in helping Ned bears fruit, but not without cost. Hanson's presence in Cornwall raises suspicion and as Demelza wrestles with how best to equip the community to look after itself, Morwenna lends herself to the cause and finds new hope.
Geoffrey Charles and Cecily's relationship continues to blossom whilst her father Ralph pursues an arrangement with George, but George's sanity continues to deteriorate and Cary struggles under mounting responsibilities. Disaster strikes and Ross and Ned rise into heroic action together.
Ross is given hope for the Despards' return to Honduras but finds his new nemesis Ralph Hanson has returned to Truro. Morwenna, still struggling with the loss of her son, starts to secretly slip away from the village, leaving Drake to wonder where she goes.
Ross returns to London with the Despards and Enyses in the hope of securing Ned and Kitty's return to Honduras at last. When the banks start issuing paper notes instead of gold, Demelza finds herself thrown into uncharted waters with her workers and the locals, which Tess and Jacka take advantage of. Morwenna tackles the grief of being parted from her son, and Drake goes to extreme lengths to ease her suffering. Meanwhile, Geoffrey Charles makes plain his intent to marry Cecily, landing them both in trouble.
George gives his maiden speech in parliament, lending support to the slavery underpinning his new business venture with Hanson. Ross must take a stand against him, but Ned's headstrong nature works against them both. As George conspires once again against his old enemy, his allies enact a plan to destroy their opposition for good.
Ross throws himself into fighting for Ned's liberty as George prepares to marry Cecily and finalise his deal with Hanson and Merceron. Geoffrey Charles is forced to make a desperate plan to elope with Cecily before the wedding, and Demelza uncovers a theft at the mine.
While Drake and Morwenna enjoy wedded bliss at last, Sam's growing attachment to Rosina comes under threat by Tess's claim to seek a purer life. With Ned's trial looming heavy and the odds stacked against them, Ross pleads for Dwight's help in a daring attempt to save his friend.
Demelza discovers the French are smuggling arms into Cornwall, but Ross is determined to take vengeance on Merceron and Hanson for framing Ned and returns to London to confront them. The Enyses follow, and Caroline must put her insecurities aside to support Kitty in her battle against Merceron, drawing perilous attention to herself in turn.
Sam continues to meet with Tess against all caution as Cecily and Geoffrey Charles plan their escape with Ross's help, but the danger that stalks the Poldarks follows swiftly.
Several months have passed as Ross gets in deeper with the French invasion force as they prepare to welcome their leader, General Toussaint. However, Ross's belief in the need for secrecy revives old insecurities and risks his friends, his marriage and his life.
Meanwhile, Morwenna and Drake are ready to welcome their baby, and George stumbles on the means to undo Ross once and for all with the help of Merceron and Hanson. With their lives are on the line, Ross and his friends must launch their most daring plan yet, to save themselves and the country from enemies both at home and abroad.