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.

The news of Demelza and Poldark's marriage spreads through the community, threatening Wheal Leisure's prospects.

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.

Whilst Ross's cooperative has some success there are hard times throughout Cornwall and typhoid sweeps through Bodmin jail and kills Jim Carter just as Ross and Dwight break in to rescue him. Dwight is also called upon to visit the injured Keren, who throws herself shamelessly at the young doctor, as Demelza notices. Untouched by privation the Warleggans throw a ball where Francis, struggling to accept his reduced circumstances, neglects Elizabeth, allowing George to step in as her escort. Ross turns up drunk and confronts Halse, the magistrate who jailed Jim and then plays cards recklessly though he does expose his opponent as a card sharp, known to the Warleggans, who are now all the more determined to break him. Further scandal is caused by the arrival of Captain Blamey, who asks Verity to run away with him.

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.

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.

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.