When neither the Whigs nor the Tories are able to form a government on their own, a fragile compromise coalition government is formed, with Plantagenet Palliser, the wealthy and hard-working Duke of Omnium, installed as Prime Minister of the United Kingdom. The Duchess, formerly Lady Glencora Palliser, attempts to support her husband by hosting lavish parties at Gatherum Castle in Barsetshire, the family's largest country house, barely used until now. Palliser, initially unsure that he is fit to lead, then grows to enjoy the high office and finally becomes increasingly distressed when his government proves to be too weak and divided to accomplish anything. His own inflexible nature does not help.
A significant sub-plot centres on Ferdinand Lopez, a financially overextended City adventurer of undisclosed parentage and doubtful ethnicity (possibly Jewish), who wins the favour of Emily Wharton. She marries him despite her father's objections in preference to Arthur Fletcher, who has always been in love with her. As in Trollope's earlier Palliser novel Can You Forgive Her?, in which the heroine also has to choose between two suitors, the enticing and charismatic suitor is revealed to have many unpleasant traits (here Lopez's ethnic background is also presented as a factor against him), and Emily soon has cause to regret her choice.
Lopez meets the Duchess at one of her parties, and Glencora unwisely encourages him to stand for Parliament. He campaigns against Arthur Fletcher, but withdraws from the contest when he sees he has no chance of winning. Lopez writes to the Duke, insisting on being reimbursed for his election expenses since the Duchess had led him to believe that he would have the Duke's endorsement (despite having his expenses already paid in full by his father-in-law).