The film opens with Prince Albert, Duke of York (later King George VI), known to his wife and family as "Bertie" (Colin Firth), the second son of King George V, speaking at the close of the 1925 British Empire Exhibition at Wembley Stadium, with his wife Elizabeth (Helena Bonham Carter) by his side. His stammering speech visibly unsettles the thousands of listeners in the audience. The prince tries several unsuccessful treatments and gives up, until the Duchess persuades him to see Lionel Logue (Geoffrey Rush), an Australian speech therapist.
In their first session, Logue requests that they address each other by their Christian names, a breach of royal etiquette: Logue tells the prince that he will be calling him Bertie from now on. At first, Bertie is reluctant to receive treatment, but Logue bets Bertie a shilling that he can read perfectly at that very moment, and gives him Hamlet's "To be, or not to be" soliloquy to read aloud, with music blaring so that he can't hear himself. Logue records Bertie's reading on a gramophone record, but convinced that he has stammered throughout, Bertie leaves in a huff, declaring his condition "hopeless." Logue gives him the recording as a keepsake.
Later that year, after Bertie's father, King George V (Michael Gambon), makes his 1934 Christmas address, he explains to his son the importance of broadcasting for the modern monarchy in a perilous international situation. He declares that Bertie's older brother, David, Prince of Wales, will bring ruin to the family and the country when he ascends the throne, and demands that Bertie train himself to fill in, beginning by reading his father's speech into a microphone for practice. After an agonizing attempt to do so made worse by his father's coaching, Bertie plays Logue's recording and hears himself reciting Shakespeare fluently, amazing both himself and the Duchess.
Bertie returns to Logue's treatment, where they work together on muscle relaxation and breath control, as Logue gently probes the psychological roots of the stammer, much to the embarrassment of the standoffish Bertie. Nevertheless, Bertie reveals some of the pressures of his childhood, among them his strict father; the repression of his natural left-handedness; a painful treatment with metal splints for his knock-knees; a nanny who favoured his elder brother, going so far as deliberately pinching Bertie at the daily presentations to their parents so that he would cry and his parents would not want to see him; unbelievably, not feeding him adequately ("It took my parents three years to notice," says Bertie); and the death in 1919 of his little brother, Prince John. As the treatment progresses, Lionel and Bertie become friends and confidants.
On 20 January 1936, King George V dies, and David, Prince of Wales (Guy Pearce) ascends the throne as King Edward VIII. However, David wants to marry Wallis Simpson (Eve Best), an American divorcée and socialite, which would provoke a constitutional crisis--the sovereign, as head of the Church of England, may not marry a divorced person.
At a party in Balmoral Castle, Bertie points out that David cannot marry Wallis. David accuses his brother of a medieval-style plot to usurp his throne, citing Bertie's speech lessons as an attempt to groom himself. Bertie is tongue-tied at the accusation, whereupon David resurrects his childhood taunt of "B-B-B-Bertie."
At his next treatment session, Bertie has not forgotten the incident. After he briefs Logue on the extent of David's folly with Wallis Simpson, Logue insists that Bertie could be king. Outraged, Bertie accuses Logue of treason and mocks Logue's failed acting career and humble origins, causing a rift in their friendship.
When King Edward VIII does in fact abdicate to marry, Bertie becomes King George VI. Feeling overwhelmed by his accession, the new king realises that he needs Logue's help, and he and the queen visit the Logues' residence to apologise. Lionel's wife is stunned to meet the royals in their modest home. When the king insists that Logue be seated in the king's box during his May 1937 coronation in Westminster Abbey, Archbishop of Canterbury Dr. Cosmo Lang (Derek Jacobi) questions Logue's qualifications. This prompts another confrontation between the king and Logue, who explains that he never claimed to be a doctor and had only begun practicing speech therapy by informal treatment of shell-shocked soldiers in the last war. When the king still isn't convinced of his own strengths, Logue sits in St. Edward's Chair dismissing the Stone of Scone as a trifle, whereupon the king remonstrates with Logue for his disrespect. The king then realises that he is as capable as those before him.
In September 1939, shortly after the United Kingdom's declaration of war with Germany, George VI summons Logue to Buckingham Palace to prepare for his radio address to the country. As the king and Logue move through the palace to a tiny studio, Winston Churchill (Timothy Spall) reveals to the king that he, too, had once had a speech impediment but found a way to use it to his advantage. The king delivers his speech as if to Logue alone, who coaches him through every moment. Afterwards, the king steps onto the balcony of the palace with his family, where thousands cheer and applaud him.
A final title card explains that during the many speeches King George VI gave during World War II (1939-1945), Logue was always present. Logue and the king remained friends, and "King George VI made Lionel Logue a Commander of the Royal Victorian Order in 1944. This high honour from a grateful King made Lionel part of the only order of chivalry that specifically rewards acts of personal service to the Monarch."