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.

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.