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 decides to reopen the family copper mine Wheal Leisure and persuades Francis to help him. Ross also escorts Francis's put-upon sister Verity to a dance, where he crosses swords with George Warleggan, annoyed that Ross borrowed capital for the mining project from a rival bank to that of his uncle. Verity meets the sea captain Andrew Blamey, who is keen to marry her but George forbids it, claiming that Blamey killed his first wife. The relationship ends when Francis challenges Blamey to a duel, in which Francis is injured, though Demelza proves her worth when she tends to him, impressing Ross.

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.

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.

With Hugh's rapidly failing health eliminating him from standing in the election, his uncle, Lord Falmouth, thanks to Caroline and Demelza, joins with Sir Francis in agreeing that Ross should oppose Warleggan as the prospective member of parliament for the district. The result is a triumph for justice but Demelza must make an admission to Ross and, for her brothers, the path of true love continues to run anything but smoothly.

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.

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.