On the eve of a general election, Henry Collingridge becomes the new leader of the Conservative Party. His Chief Whip is Francis Urquhart, an icy, intelligent man who is judged to be reliable but a little old-fashioned. Collingridge narrowly wins the election but reneges on his promise to promote Urquhart who, despite his hidden rage, pledges his support.

Urquhart's wife urges him to depose the PM and win the office for himself. Fuelled by fury and hunger for revenge, the Chief Whip resolves to follow her suggestion. He begins by manipulating a young journalist named Mattie Storin, leaking her information that damages Collingridge. Despite the injurious nature of the material he passes her, Urquhart succeeds in making his intentions appear honourable, as if he acts solely for the good of his party. He also blackmails Roger O'Neall, an alcoholic drug abuser who ranks as one of the Conservatives' top media executives. Urquhart has noted certain financial irregularities in his expenses claims and exploits this knowledge, forcing O'Neall into lending him unstinting support.

Urquhart's early maneuverings leave the PM isolated, but a scandal he concocts concerning Collingridge's brother forces his resignation. Feigning humility, the Chief Whip announces his intention to stand for the premiership. He immediately exploits sex scandals, leaks and anonymous threats to discredit and discourage his opponents, ultimately emerging as the clear favourite to succeed Collingridge.

However, O'Neall is growing unstable, and Storin, now Urquhart's doting mistress, begins to suspect she has been a pawn. Before O'Neall can reveal the truth of the duplicitous plotting, Urquhart laces his supply of cocaine with rat poison and he dies.

On the verge of becoming Prime Minister, Urquhart waits alone in the House of Commons' roof gardens, where he is joined by Storin. She confronts him with her suspicions and, aware she has become a potential liability, he throws her over the wall to her death. An unseen bystander surreptitiously pockets her dictation machine which appears to have been recording her final conversation.

En route to Buckingham Palace, where the Queen will invite him to lead her government, the radio broadcasts Urquhart's shallow, distancing eulogy for the woman he murdered.