On the eve of her husband's trial Demelza ignores Ray Penvenen's advice and approaches Judge Lister for mercy but her plan is scuppered by George Warleggan. Warleggan is convinced that Ross will be found guilty but Ross is an immensely popular defendant and despite the intervention of Demelza's father and Warleggan's paid prosecution witnesses his own excellent defence, aided by the testimony of the captain whose life he saved, Dwight and surprisingly Jud, leads to an acquittal. Dwight also attends Francis following his failed suicide bid after his belief that Elizabeth had an affair with Ross and gets closer to Caroline, who is not sorry when outsider Trevaunance's election as MP causes a riot. Warleggan however is still desperate to acquire Ross's mine by any means whilst Demelza and Ross consider having another baby.

Ross chairs the meeting for his mine's share-holders, where expansion of Wheal Leisure is approved but Warleggan's spy Tankard is also present and Warleggan's heavies attack Jud for failing to discredit Ross at his trial after their payment to commit perjury. To remain solvent the Poldarks are forced to sell many of their possessions and livestock, causing Demelza to keep her pregnancy secret from her husband. However, following the attack on Jud, Ross realizes Warleggan has schemed to keep Francis and himself at odds and is reconciled with his cousin at a family party, after which Demelza reveals her pregnancy to a far from displeased Ross.

An epidemic sweeps through the county and Dwight is grateful to Caroline for providing him with a cart-load of fresh oranges to combat it. Ross meanwhile is thwarted in his efforts to expand the mine, thanks to Warleggan and his proxy, Tankard. To provide finances he strikes a deal with a local smuggler to use his beach to land cargo on, much to Demelza's annoyance. Ross also goes into partnership with Francis, despite Warleggan's attempts to split them up, and they open up the mine gaining access to Wheal Grace. Demelza gives birth to a baby boy, Jeremy, which goes some way towards reconciling Francis with Verity.

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.