Ross Poldark returns from fighting in the American Revolution to his native Cornwall, finding his fiancée Elizabeth Chynoweth about to marry his wealthy cousin Francis and his home in ruins after his father's death. Ross is a reluctant wedding guest, along with George Warleggan, nephew of a prosperous banker, and his uncle Charles proposes financing him to move to London. Ross, however, stays in Cornwall, rebuilding his home. Whilst at the local market he rescues the waif-like Demelza Carne and takes her home to work for him. Her bullying father arrives with his sons to reclaim her but Ross's tenants turn up to help him and fight them off.

Ross reopens Wheal Leisure, drawing his work force from Charles' mine and winning the respect of his tenants. Whilst he and Demelza are at a local wedding Elizabeth gives birth to a son, Geoffrey Charles. Ross attends the christening , noting not only the growing friendship between George Warleggan and Francis but overhearing gossips speculate on his relationship with Demelza. He is genuinely impressed by her efficiency and starts to confide in her. One of his workers, young father Jim Carter, is taken to court for poaching and Ross speaks on his behalf but fails to prevent the boy being sent to prison. Drunk and miserable he returns home and ends up in bed with the comforting Demelza. Next day he is visited by a sympathetic Elizabeth, clearly unhappy in her marriage. Seeing them together Demelza prepares to leave but Ross urges her to stay and they get married.

Wedding bells ring as miner Mark Daniel marries visiting actress Keren Smith whilst Demelza and Ross have their own celebration as their daughter Julia is born, Ross not allowing Demelza's father's unwelcome visit to the christening party to ruin matters. Young Dr Dwight Enys comes to the area and is befriended by Ross. He is progressive and philanthropic, which annoys the curmudgeonly incumbent Dr Choake. With widespread poverty and mine closures all around Ross proposes that the pit owners form a collective with their own smelting company, to beat the Warleggans' monopoly, and presses ahead with the scheme. Francis opts out and is subsequently ruined as his mine is forced to close. To repay Verity for her kindness Demelza reunites her with Andrew Blamey in town, where they witness a food riot by starving locals.

Francis wrongly blames Ross for helping Verity elope with Andrew Blamey. Meanwhile, Mark suspects Dwight and Keren of having an affair, starting chain of events which will have tragic consequences and threaten the safety of eveyone in Nampara.

Ross faces ruin as, along with the rest of his collective, he must sell to the Warleggans' company at a nominal price. At the same time the disease known as putrid throat sweeps through the county, claiming many lives and Dwight, still contrite over Keren, throws himself into trying to treat it. Elizabeth and her family are stricken and Demelza goes to help them but infects herself and her daughter with disastrous results. Divine justice appears to intervene as the Warleggans' ship and its cargo founder on the rocks and Ross leads the villagers in collecting its spoils from the beach. Demelza recovers from her sickness, only to face another setback to her marriage.

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.

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.

The mine is at last bringing Ross wealth but Demelza, still slow to forgive, goes to stay with new mother Verity. An angry Ross gives the snide Warleggan a beating after his suggestion that Ross is cheating Elizabeth's son out of his inheritance before deciding to reenlist to fight in the Napoleonic wars. Dwight also signs up as a naval surgeon but is reunited with Caroline before he sets sail. Warleggan builds fences around his property, cutting off the public right of way and Demelza is one of the casualties of his hired thugs. Irate locals march on his house, Trenwith, vowing to burn it to the ground, but Ross dissuades them to spare them prosecution before he is reconciled with Demelza.

Whilst anxious to build their own Methodist chapel, the Carne brothers work well at the mine, though they defy Ross by entering Warleggan's estate, where Drake meets and is captivated by Morwenna. Along with Caroline, Verity attends Valentine's christening where she unsuccessfully attempts reconciliation between the Warleggans and Ross. Both women are alarmed to hear their husband's ships are missing at sea, but Verity's spouse Andrew is found alive and well. Despite unexpected support from the Reverend Halse, Ross refuses to put himself forward as magistrate replacing the late Ray Penvenen, giving Warleggan the victory. Later Ross hears that Dwight has been cast ashore in revolutionary France and, with his friend Tholly Tregirls, sets out to rescue him.

Tholly takes Ross to a French port where republican soldiers are executing their prisoner. They meet Jacques Clisson, who gives them information about Dwight's incarceration but, threatened with prison himself, Ross returns home without his friend. Banned by Warleggan from using his church, the Carne boys find a venue for their Methodist meeting house, but are again thwarted by Warleggan through a legal loop-hole and are ultimately saved by Demelza, who allows them to use a barn on Ross's land. The Warleggans move to Truro, where even Elizabeth is shocked by the immoral social climbing of her newly appointed magistrate husband, and starts to feel isolated, but their absence leaves Morwenna, encouraged by Aunt Agatha and Geoffrey Charles, to continue meeting Drake.

Guests come to Ross and Demelza's daughter Cowance's christening. Morwenna and Geoffrey Charles are called away for Christmas. George plots to marry Morwenna off for political gain, but her heart belongs to Drake. Ross, Demelza, and Caroline find a way to feed the poor. Dwight continues to suffer in French prison.

Whilst Ross returns home a hero and Morwenna and Drake continue to meet in secret Dwight, severely traumatized by his experiences, frightens Caroline by his seeming indifference to her. Ross brings Hugh Armitage who succeeds in helping Dwight get rid of his demons. Aware of the spirited Geoffrey Charles' loyalty to the clandestine lovers Warleggan arranges to send him to boarding school, but when Geoffrey Charles gives Drake a parting gift, Warleggan uses it to accuse Drake of theft. He is eventually released, and the heavy price for his acquittal is that Morwenna must marry Whitworth.

When French ships are seen off the coast, Sir Francis appoints Ross to lead the volunteers ready to repel any invasion, to the annoyance of Warleggan. George is further angered when Sir Francis criticizes his use of his parliamentary privilege for his own benefit. He vents his rage on Drake, having him brutally assaulted and his forge destroyed. Demelza, disappointed by Ross's apparent failure to avenge her brother, spends more time with Hugh, while Ross quells a rebellion against Warleggan by agreeing to stand for parliament himself. Rowella finds herself pregnant by Whitworth, and blackmails him into paying a young local, Arthur Selway, to marry her, giving Morwenna the strength to rebel against him. Elizabeth also revolts against her cruel husband, who begins to lose his cocky self-assurance, whilst Demelza and Ross go to bed with doubts on either side.

Times are hard for the locals as grain is being sold to the highest bidders abroad. A corn riot breaks out in Truro, and the Carne boys, innocent bystanders, are among those arrested. MP George Warleggan manipulates the trial against them, but Ross does his best to save them, impressing Sir Francis Basset, who wants him to stand in the upcoming election. Demelza is torn between her admirer, Hugh Armitage and Ross, but she and Ross look to rebuild their marriage. Caroline has good news for Dwight, while Morwenna continues to endure life with her husband.

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.