When a smuggler is caught magistrate Francis cleverly ensures a lenient sentence but Ross suspects one of his miners, Nick Vigus, as an informer. Francis is also loyal to his cousin when they meet Warleggan at a party, where Elizabeth admits that she still has affection for Ross and Caroline jilts Trevaunance before Dwight reveals his feelings for her. Francis discovers copper in the mine but his jubilation is cruelly curtailed whilst Ross once more finds himself at the mercy of Warleggan, who has obtained his promissory note and has a financial hold over him.

A cheerless Christmas looms for the Poldarks with Francis drowned in the mine having only found fool's copper and Ross facing debtor's prison if he cannot pay Warleggan. Ross is also anxious to expose the traitor and agrees to a dangerous scheme to trap them whilst Demelza is troubled by his frequent visits to console Elizabeth - though Warleggan is quick to ingratiate himself with the widow. Ray Penvenen accuses Dwight of fortune hunting and tries to ban him from seeing Caroline but she is far too perceptive and realistic to give up the man she loves. She also bails out Ross, whose tenacity she admires, in his hour of need - whilst seeking to remain anonymous.

Ross joins a smuggling trip in the Isles of Scilly in order to meet Mark Daniel but, in his absence, George moves in on Elizabeth. Caroline convinces Dwight they must elope for a new life in Bath, and Captain McNeil plans to ambush the smugglers on their return.

Ross and Dwight stand trial for their part in the smuggling. A new tin lode gives Ross renewed hope in Wheal Grace, but it is George who is there for Elizabeth when her mother falls ill and deals Ross a bitter blow.

Ross is now member of parliament for Truro and making compassionate speeches opposing slavery and the use of child workers. Inevitably Warleggan, smarting from his defeat, links up with the disreputable Monk Adderley for revenge on Ross and a seat of his own whilst Whitworth seeks another way to discredit him. Returning to Cornwall, where Caroline has given birth to a daughter Sarah, Ross is called upon to save an ailing mine and also pays a visit to Elizabeth.

Caroline and Dwight suffer a family tragedy, prompting her to take unusual action. Dwight also confounds Whitworth's attempt to commit Morwenna to an asylum, to allow him to continue his affair with her sister Rowella and proves himself a hero when blasting causes heavy flooding at the mine. This last event causes Ross to doubt a return to parliament but is persuaded by Dwight to change his mind. Warleggan also enables his political ambitions to come to crooked fruition.

In parliament Ross achieves some success on behalf of his constituents. He also meets Caroline, who has fled her recent tragedy and who introduces him to Monk Adderley. Back in Cornwall there is famine and sickness. However Morwenna and Drake are afforded a second chance when Whitworth's sins catch up with him although Drake, having broken off his wedding to Rosina Hoblyn, comes under suspicion.

Warleggan is triumphant to discover that the late Nathaniel Pearce embezzled money from Pascoe's, the bank used by Ross. With Ross in London - impressing prime minister Pitt - Demelza attempts to save the bank but fails and Warleggan takes full advantage to plot further. Drake is dissuaded from leaving the area by Sam but Morwenna still rejects him whilst Caroline returns from London with Ross for the opening of Dwight's new infirmary. Here Ross makes an impassioned speech which leads to the bank being saved. He also has a surprise for Demelza whilst Elizabeth has news of her own.

Now suspicious about Valentine's paternity after Geoffrey Charles' observation, Warleggan is cold towards Elizabeth and, to force an early delivery of her unborn child to allay further suspicions about Ross being Valentine's father, she takes drastic measures - which do not end well. Drake, however, is overjoyed when Morwenna agrees to marry him whilst Lord Falmouth refuses to accept Ross's resignation from Parliament because of his duel with Monk Adderley.

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, but George's sanity continues to deteriorate and Cary struggles under mounting responsibilities. 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 spring into action together.

Ross and Demelza are caught in the middle as Ralph Hanson tries to ruin Ned Despard and rebellion in Cornwall is stirred.

Ross tries to persuade the government to reinstate Ned as governor of Honduras, but Ned voices controversial views. The miners are distrusting of paper money. Morwenna visits John Conan. Demelza and Sam try to raise funds for a school.

Ned and Kitty face dire consequences and Ross attempts to help them. Cecily and Geoffrey Charles seek a life together against her father's wishes.

Demelza discovers the French are smuggling arms into Cornwall, but Ross is determined to take revenge on Merceron and Hanson for framing Ned and returns to London to confront them.

Ross and his friends must launch their most daring plan yet to try to save themselves and the country from enemies both at home and abroad.