In Terry Pratchett's Wyrd Sisters, wicked Lord Felmet kills the king of Lancre and takes over the throne. Three witches, Granny, Nanny, and Magrat, who have secreted away the heir to the throne, must overthrow Lord Felmet by revealing the truth. They turn Lord Felmet's lie-filled play around, making the players speak the truth and freeing the kingdom for a new ruler.

When the king is assassinated at the beginning of the novel, the coven of three witches unwittingly winds up with the infant heir to the throne. Granny, Nanny and Magrat, after defeating the usurper's assassins, entrust the baby to traveling thespians. Lord Felmet, usurper of the throne, is disturbed by reports of tax-exempt witches in his kingdom. He orders tax collectors to visit the witches and collect. The taxmen, however, are no match for the witches' mother-like authority and are unable to complete their task.

Lord Felmet's Fool convinces his majesty to consider a war of words, undermining the witches' authority through rumor and implication. The spirit of the kingdom, however, sensing Felmet's disdain for it, rebels against the new king with an earthquake. The witches, too, are contacted by various representatives of the kingdom, both man and animal, and asked to intercede against Lord Felmet. The witches are reluctant to meddle. Meanwhile, the Fool and Magrat, youngest of the witch coven, meet by chance and fall in love.

When the ghost of the dead king lures Nanny to the castle, the remainder of the coven is forced to rescue her. This puts the witches face to face with the usurper Felmet, whereupon they realize they cannot interfere without undermining the authority of the crown. The Fool's campaign of rumors, however, has taken its toll. Open disrespect from the citizenry pushes Granny to drop her compunction against meddling. The other witches follow suit.

With the Fool's assistance, Lord Felmet hatches a plot to rewrite history via a play, which will tell the "truth" about the old king and his untimely death. Granny decides, meanwhile, that she will see the king's heir, Tomjon, seated upon the throne. Granny, with the help of Nanny and Magrat, casts a spell that hurls the entire kingdom forward in time fifteen years.

Lord Felmet sends the Fool to Ankh-Morpork to find a playwright. There the Fool encounters the grown-up Tomjon and the theatre troupe to which he belongs. Without realizing that Tomjon is the prince, the Fool hires the playwright Hwel to write the play and stage it in Lancre. Seeing via crystal ball that Tomjon is preparing for a trip to Lancre, the witches assume he intends to recover the throne.

Hwel stages the play in Lancre, but the production is a disaster. The witches interfere, forcing the actors to speak the truth. The Fool, moving to protect Magrat from arrest, bears witness against the lord, accusing him of murder. Lord Felmet becomes entirely unhinged, flees, and dies to an accidental fall. Lady Felmet, the instigator behind Lord Felmet, is arrested and imprisoned.

Tomjon refuses the throne of Lancre. He wishes to return to his theatre troupe in Ankh-Morpork. The Fool, who happens to be Tomjon's illegitimate older brother Verence, ascends to the throne instead and continues his relationship with Magrat. Nanny and Granny reveal to Magrat that, while Verence is indeed Tomjon's half-brother, they share a mother, not a father. Magrat intends to keep it a secret.