Ross prepares for his trial as Demelza and Elizabeth try to help his cause. George does everything in his power to harm Ross, Francis wrestles with his conscience, and Dwight clashes with an entitled heiress, Caroline Penvenen.
As the Poldarks look to rebuild their marriage and the Enyses to increase their family, Cornwall struggles under the Warleggans, whose power threatens all they hold dear.
The Poldarks determine to get their relationship on track and the Enyses announce they're expecting a baby. Hugh's worsening health threatens his chance to stand in the election, and the prospect of him remaining in Cornwall adds pressure to the Poldarks' fragile marriage. At the Sawle Feast, Tom Harry challenges Sam to a wrestling match. When Emma offers herself as the prize, Sam accepts.
Ross faces the challenge of Parliament and the Warleggans refuse to accept defeat. In Cornwall, Demelza contends with a failing mine and fixing Drake's broken heart. Dwight and Caroline welcome a baby daughter. Ossie meanwhile tends to a dying parishioner and George hosts an extravagant party at Trenwith.
The Poldarks look forward to life together in peace, but a plea from his old army colonel, Ned Despard, compels Ross to the capital. Arriving in London, he finds a city gripped with paranoia, and is determined to save Ned from the conspiracy that imprisoned him. As Demelza manages their affairs in Cornwall she encounters disenchantment and a new opponent, Tess. Meanwhile, George struggles to engage with the world after Elizabeth’s death, but enters into an alliance with the merchant behind Ned’s imprisonment, Ralph Hanson, who is involved in the slave trade.
Demelza and the children join Ross in London, but his dogged pursuit of Ned’s vindication finds them caught in a dangerous web. An ennobled George pursues the fruits of his new alliance, but his lingering grief has unexpected consequences. As Ned confronts the man responsible for his downfall, Demelza fears they are entangled with forces beyond their control, and Dwight’s expertise on insanity unwittingly place him and his friends on shaky ground. Meanwhile, Drake and Morwenna continue tentative steps forward in their relationship, Valentine’s spirit grows wilful in neglect, and Geoffrey Charles learns he must tread carefully if he’s to gain Cecily’s father’s favour. Ross tries to find a man who may hold the key to clearing Ned’s name, but in championing Ned’s cause, Ross finds himself in a compromising situation that will shadow him wherever he goes.
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. Bored of being neglected, Valentine decides to break out of Trenwith in search of new friends. Meanwhile, the Enyses look to challenge the county’s high society by hosting a ball to introduce the Despards, before disaster strikes and Ross and Ned must 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. Demelza continues to help the community help itself, and Dwight gains unexpected patients but his absences from home have unforeseen consequences. Morwenna, still struggling with the loss of her son, starts to secretly slip away from the village, leaving Drake to wonder where she goes. Sam and Rosina become close as Rosina starts to lend a hand with the school, Geoffrey Charles and Cecily continue meeting in secret but discovering the nature of Ralph and Cary's business plan leaves them in turmoil. Meanwhile, Ralph enlists some help from Tess, and disgruntled locals to set a plot in motion to undo the Despards and the Poldarks in turn.
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.
