Cornwall in England, towards the end of the eighteenth century. Ross Poldark returns from the American War of Independence to find his beloved, but finds only one world in ruins: his father died, the family mine closed and his companion committed to marry his cousin. Fortunately for Poldark, nothing is lost...
Caroline convinces Dwight they must elope for a new life in Bat, as Ross sails with Trencrom to meet with Mark Daniel. Caroline is forced to pull her plan forward but it is threatened when Rosina dislocates her knee again. Dwight believes he can treat her and still rendezvous with Caroline; however, he and Demelza become suspicious that Charlie Kempthorne is the informant when they learn he has avoided the smuggling run. Dwight confronts Charlie, and a fight ensues. Demelza realises Ross will be walking into an ambush and attempts to warn the boat, but McNeil and his soldiers arrive at Nampara, leaving her under guard. Dwight incapacitates Charlie and escapes as Caroline sneaks out of Killewarren to meet him. Dwight attempts to warn Ross but is too late. Will Ross find a way to evade the soldiers? Having paid men to harass Trenwith with the aim of driving Elizabeth to him for protection, George arrives at her home and posts his men to guard Elizabeth, who is both shocked and grateful for George's attention. Dwight is released by the soldiers, but finds Caroline has already returned to London
Demelza refuses to speak to Ross and expects him to return to Elizabeth, abandoning her and Jeremy. Elizabeth and Agatha believe the same and Elizabeth writes to George to postpone the wedding. George rides to Trenwith to make Elizabeth agree to another date, ensuring his plans are not usurped. In Truro, Tonkin invites Ross to join him in a prospering shipbuilding partnership. Meanwhile, Demelza accepts Sir Hugh Bodrugan’s invitation to his ball, where she finds Captain McNeil, inviting him to her room, having justified the possibility of adultery as something Ross has driven her to. Ross finds Demelza on Hendrawna Beach but hasn’t the words to build a bridge between them. Ross is beckoned to the mine where they have cleared the collapse and come upon the richest lode of tin Henshawe has ever seen - may Ross soon may be free of debt?
Elizabeth tells George that she is carrying his child and George continues to mark his territory by erecting fences around Trenwith, denying commoners passage across his property. Dwight learns of Caroline's new engagement and enlists in the navy. Ross decides he must mend the rift between Caroline and Dwight before it is too late. Ross is called to Trenwith by George, suspicious that the purchase of Geoffrey Charles' share in the mine was fraudulent. Pascoe reveals to Ross that George will seek to bankrupt him through court proceedings. Demelza is assaulted for trespassing on Warleggan land provoking the villagers to march on Trenwith with vengeance in mind - will Ross be able to disband the mob? Elizabeth, meanwhile, is forced to face the possibility that her pregnancy might not, in fact, be the result of her union with George.
As Elizabeth becomes more ambivalent about Valentine, George courts Sir Francis Basset for the vacant magistracy position. Ross is reintroduced to Tholly Tregirls, an old friend of his father's and a ne'er-do-well who has returned to area. Verity returns to Trenwith, awaiting word from her husband, Andrew, whose ship is en route to Lisbon, while Elizabeth feels the sting of Morwenna and Geoffrey Charles's connection.
Ross is ordered to leave Roscoff under threat of execution. He travels with Tholly to meet his contact in France, but finds information about Dwight's fate comes at a high price. Sam leads a congregation of methodists in protest outside the church - only to face broken promises from George, who revokes their lease on the meeting house, which leads them to turn to Elizabeth for help. Geoffrey Charles has some words of advice for Drake, who has shared a moment of intimacy on the beach with Morwenna.
A grain ship is raided by starving villagers, and George arranges a harsh punishment for the desperate peasants while throwing a lavish Christmas party to impress the local gentry. Caroline hears news that the Admiralty has arranged for the release of the British officers, but Ross is not convinced. Demelza gives birth to a daughter, and warns Morwenna against a relationship with Drake.
The French Royalists plan an invasion, which Caroline hopes will lead to Dwight's release. However, Ross is convinced this will only make matters worse, and recruits a band of allies, including Tholly, Zacky, Drake and Henshawe, to help him free Dwight from prison. George schemes to further his political ambitions, and when he learns of Ross' plan, he uses it as a means to discredit his rival, accusing him of recklessness - a charge that may prove true when the rescue mission proves to have fatal consequences.
The Poldarks learn that their friend Dr Enys needs immediate care, suffering distress following his internment in France. In reaction to Ross' rescue of Hugh Armitage, George desperately needs to secure a connection to the nobility and so plans to force Morwenna's marriage to Osborne. Geoffrey Charles gives Drake his bible as a gift and naively reveals that Morwenna is to be wed. Drake confronts Morwenna and she agrees to marry him instead. Morwenna returns to Trenwith to tell George and Elizabeth that she will refuse Ossie's offer of marriage. Out of spite, George has Drake arrested for the theft of Geoffrey Charles' valuable bible, a crime for which Drake will hang.
George learns of a vacant seat in Parliament for a Member of Truro and decides to pursue it. Elizabeth and Agatha prepare for Agatha's 100th birthday party, to George's chagrin, and Caroline hosts a fake wedding reception to which the great and good are invited. Meanwhile, there is a new arrival at the Whitworth vicarage.
Ross is adamant George will lose in the election and determines to get a headstone made for Agatha's grave, something Trenwith has neglected. George, meanwhile, questions Dwight over Valentine's legitimacy and - paranoid and jealous - shuts himself to Elizabeth, as Rowella begins to seduce Osborne Whitworth and Demelza receives tokens of Hugh's affection. Sam continues to pine after Emma, as Dwight instructs Osborne to forego intercourse with Morwenna and Cary informs Lord Falmouth that the Warleggans will be opposing Falmouth in the election. Ross returns from his mine to discover Hugh Armitage at Nampara with Demelza - and this time Hugh's attention does not go unnoticed..
The Poldarks determine to get their relationship on track and the Enyses announce they're expecting a baby. Hugh's worsening health threatens his chance to stand in the election, and the prospect of him remaining in Cornwall adds pressure to the Poldarks' fragile marriage. At the Sawle Feast, Tom Harry challenges Sam to a wrestling match. When Emma offers herself as the prize, Sam accepts.
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.
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.
The Poldarks look forward to life together in peace, but a plea from his old army colonel, Ned Despard, compels Ross to the capital. Arriving in London, he finds a city gripped with paranoia, and is determined to save Ned from the conspiracy that imprisoned him. As Demelza manages their affairs in Cornwall she encounters disenchantment and a new opponent, Tess. Meanwhile, George struggles to engage with the world after Elizabeth’s death, but enters into an alliance with the merchant behind Ned’s imprisonment, Ralph Hanson, who is involved in the slave trade.
Demelza and the children join Ross in London, but his dogged pursuit of Ned’s vindication finds them caught in a dangerous web. An ennobled George pursues the fruits of his new alliance, but his lingering grief has unexpected consequences. As Ned confronts the man responsible for his downfall, Demelza fears they are entangled with forces beyond their control, and Dwight’s expertise on insanity unwittingly place him and his friends on shaky ground. Meanwhile, Drake and Morwenna continue tentative steps forward in their relationship, Valentine’s spirit grows wilful in neglect, and Geoffrey Charles learns he must tread carefully if he’s to gain Cecily’s father’s favour. Ross tries to find a man who may hold the key to clearing Ned’s name, but in championing Ned’s cause, Ross finds himself in a compromising situation that will shadow him wherever he goes.
Ross's tenacity in helping Ned bears fruit, but not without cost. Hanson’s presence in Cornwall raises suspicion and as Demelza wrestles with how best to equip the community to look after itself, Morwenna lends herself to the cause and finds new hope. Geoffrey Charles and Cecily’s relationship continues to blossom whilst her father Ralph pursues an arrangement with George, but George’s sanity continues to deteriorate and Cary struggles under mounting responsibilities. Bored of being neglected, Valentine decides to break out of Trenwith in search of new friends. Meanwhile, the Enyses look to challenge the county’s high society by hosting a ball to introduce the Despards, before disaster strikes and Ross and Ned must rise into heroic action together.
Ross is given hope for the Despards' return to Honduras but finds his new nemesis Ralph Hanson has returned to Truro. Demelza continues to help the community help itself, and Dwight gains unexpected patients but his absences from home have unforeseen consequences. Morwenna, still struggling with the loss of her son, starts to secretly slip away from the village, leaving Drake to wonder where she goes. Sam and Rosina become close as Rosina starts to lend a hand with the school, Geoffrey Charles and Cecily continue meeting in secret but discovering the nature of Ralph and Cary's business plan leaves them in turmoil. Meanwhile, Ralph enlists some help from Tess, and disgruntled locals to set a plot in motion to undo the Despards and the Poldarks in turn.
Ross returns to London with the Despards and Enyses in the hope of securing Ned and Kitty’s return to Honduras at last. When the banks start issuing paper notes instead of gold, Demelza finds herself thrown into uncharted waters with her workers and the locals, which Tess and Jacka take advantage of. Morwenna tackles the grief of being parted from her son, and Drake goes to extreme lengths to ease her suffering. Meanwhile, Geoffrey Charles makes plain his intent to marry Cecily, landing them both in trouble. George gives his maiden speech in parliament, lending support to the slavery underpinning his new business venture with Hanson. Ross must take a stand against him, but Ned’s headstrong nature works against them both. As George conspires once again against his old enemy, his allies enact a plan to destroy their opposition for good.
Ross throws himself into fighting for Ned’s liberty as George prepares to marry Cecily and finalise his deal with Hanson and Merceron. Geoffrey Charles is forced to make a desperate plan to elope with Cecily before the wedding, and Demelza uncovers a theft at the mine. While Drake and Morwenna enjoy wedded bliss at last, Sam’s growing attachment to Rosina comes under threat by Tess’s claim to seek a purer life. With Ned’s trial looming heavy and the odds stacked against them, Ross pleads for Dwight’s help in a daring attempt to save his friend.
Demelza discovers the French are smuggling arms into Cornwall, but Ross is determined to take vengeance on Merceron and Hanson for framing Ned and returns to London to confront them. The Enyses follow, and Caroline must put her insecurities aside to support Kitty in her battle against Merceron, drawing perilous attention to herself in turn. Sam continues to meet with Tess against all caution as Cecily and Geoffrey Charles plan their escape with Ross's help, but the danger that stalks the Poldarks follows swiftly.
Several months have passed as Ross gets in deeper with the French invasion force as they prepare to welcome their leader, General Toussaint. However, Ross's belief in the need for secrecy revives old insecurities and risks his friends, his marriage and his life. Meanwhile, Morwenna and Drake are ready to welcome their baby, and George stumbles on the means to undo Ross once and for all with the help of Merceron and Hanson. With their lives are on the line, Ross and his friends must launch their most daring plan yet, to save themselves and the country from enemies both at home and abroad.