As the Poldarks look to rebuild their marriage and the Enyses to increase their family, Ross watches Cornwall suffer under the power of Truro's MP, George Warleggan, whose authority threatens all he hold dear. Prime Minister Pitt calls an election, and Hugh is set to rival George in the upcoming election. A corn riot erupts in Truro, and Zacky's son, Jago, leads the charge. Demelza's infidelity leaves her torn between her husband and a lovesick Hugh.
The Poldarks determine to get their relationship back on track and the Enyses announce they're with child. Hugh's worsening health threatens his chance to stand in the election however, and the prospect of him remaining in Cornwall adds pressure to the Poldark's fragile marriage. At the Sawle Feast, Tom Harry challenges Sam to a wrestling match. Meanwhile, Cornwall's election looms and the Poldarks face difficult decisions, for the outcomes could change not only their lives, but the future of Cornwall itself.
Ross faces the challenge of Parliament and the Warleggans refuse to accept defeat. In Cornwall, Demelza contends with a failing mine and fixing Drake's broken heart. Dwight and Caroline welcome a baby daughter. Ossie meanwhile tends to a dying parishioner and George hosts an extravagant party at Trenwith.
Ross is reunited with his family at home, but tragedy strikes in Cornwall. Dwight tells Caroline some devastating news about Sarah. Ossie is seen visiting Rowella by her husband Arthur, who grows suspicious. Emma returns to give Sam her answer, whilst Drake is persuaded into courting Rosina.
George increases the strain on the struggling poor; To his frustration, Ross's efforts in Westminster meet successive failures, driving Ross to make a risky gambit. The adversity the Poldarks face, however, may not be confined to Cornwall alone.
As Dwight helps in the founding of Truro's infirmary, George finds the opportunity to ruin a business rival and the Poldarks in the process.
The Poldarks, Enyses and Warleggans enjoy London's distractions. George dotes on Elizabeth, delighting at the prospect of an heir and announcing plans to host a charity ball, but their domestic bliss soon comesunder threat. At the Warleggan ball Adderley takes a fancy to Demelza, betting George he'll cuckold Ross within a month, which gives rise to deadly complications.
Elizabeth obtains a tonic designed to induce an early labour, she hopes to persuade George that Valentine is his child once and for all, but the Warleggan's rift is plain as they return to Cornwall. Ross returns to Cornwall to mend things with Demelza, while Demelza helps Morwenna and Drake build a future.