As the first season of Deadwood opens, former marshall Seth Bullock and his friend Sol Star are starting a new hardware business in the Deadwood camp. Business is booming as there is a neverending stream of amateur gold miners coming into town.
Points of interest in Deadwood include The Gem, owned by Al Swearengen, which employs both bar staff and prostitutes, the newspaper office, and The Grand Hotel. A gambling establishment, the Bella Union, opens to compete with The Gem for customers.
One of the wealthy residents of The Grand Hotel, Brom Garrett, is swindled by Swearengen in a gold claim con. After Garrett confronts Swearengen, he is killed by one of Swearengen’s men. Shortly before Garrett’s death, the fraudulent gold claim in discovered to in fact be a true gold mine.
Garrett’s widow, who is dealing with an addiction to the narcotic laudanum, seeks help from Bullock about what to do with her new wealth. She also takes in a young girl, Sofia Metz, after the girl’s family is murdered on the road.
After Wild Bill Hickok is killed during a card game, Swearengen convinces a local magistrate to acquit the killer, in order to avoid attention from Deadwood outsiders. A smallpox outbreak segregates the camp, and the community attempts to organize a government as the local leaders eye a possible annexation by the Dakota territories.
As it becomes more and more evident that Swearengen’s corruption is ruling the town, Bullock volunteers to become the new sheriff in Deadwood.
Bullock and Swearengen fight in the streets after Swearengen confronts Bullock about his affair with the widow Alma. Alma and Bullock decide to stop seeing each other. Later, Alma is forced to marry another man to evade a scandal after she realizes she is pregnant with Bullock’s child.
Two Deadwood residents, Joanie and Maddie, open their own brothel, the Chez Ami. Francis Wolcott, who works for the famously wealthy businessman George Hearst, frequents Chez Ami and murders Maddie and two of the prostitutes. These murders are covered up by the owner of the rival brothel the Bella Union, while Joanie sends the rest of the women away to protect them.
Alma considers buying The Grand Hotel with her gold money, but opens a bank in Deadwood instead. Swearengen is sidelined in his hotel for an extended time while trying to pass kidney stones.
Bullock’s young son is trampled by a horse and dies. Swearengen and Mr. Wu join forces against Wolcott, who has caused the death of several Chinese prostitutes. When Wolcott’s boss, George Hearst, arrives in Deadwood, he fires Wolcott for his evil deeds. However, Hearst is blackmailed by Bella Union owner Tolliver, who suggests he can prove that Hearst knew of Wolcott’s murdering ways much earlier.
As season two ends, the town is annexed by the Dakota Territory, after much discussion about its future.
In season 3, Bullock is in a heated race for sheriff and Hearst tries to convince Alma to sell him her gold claim.
Several miners are murdered by Hearst’s men after they attempt to form a union. Swearengen’s attempts to curb Hearst’s powergrab result in Swearengen getting his finger cut off by one of Hearst’s men.
Alma refuses to sell her claim to Hearst and has an abortion after her health declines. Her husband Ellsworth agrees to a separation after he discovers she has gone back to abusing laudanum due to her marital unhappiness.
After Bullock sends Hearst to jail for a night, Hearst continues to create chaos in the town. Ellsworth is killed by one of Hearst’s men. Alma is shot. She survives, but agrees to sell her claim to Hearst to avoid further bloodshed. After the prostitute Trixie unsuccessfully tries to kill Hearst, her employer Swearengen successfully saves her from Hearst’s wrath by convincing him Trixie is dead.
Hearst leaves Deadwood as season 3 ends.