The full story
Three years after WandaVision, Agatha Harkness is still trapped in Westview under Wanda's spell, believing she's Agnes O'Connor, a detective in a crime show. A teenager breaks into her house looking for "the Road," and breaks the spell. Agatha wakes with no powers, which Rio Vidal, a witch with a long history with her, would happily use as her chance to kill her, and the Salem Seven are coming too.
The Teen wants to walk the Witches' Road, a legendary gauntlet that gives whoever survives it what they most desire. A sigil on him hides everything about who he is, even his name. For Agatha the prize is her power back, and walking the Road takes a coven. They recruit Lilia Calderu, a centuries-old Sicilian diviner; Jennifer Kale, a potions expert whose powers have been bound; and Alice Wu-Gulliver, a protection witch whose family is haunted by a curse. Unwilling to ask Rio, Agatha makes Sharon Davis, Westview's "Mrs. Hart," a gifted gardener with no magic at all, their green witch. They sing open the door and escape the Salem Seven onto the Road.
Each trial is built around one of them. A beach house serves poisoned wine, and Sharon dies of it. The spell for a new green witch summons Rio. A 1970s recording studio calls up the demon behind the Wu family's curse, and Alice kills it. A 1980s cabin lets Agatha's dead mother Evanora possess her; Alice drives her out, and Agatha drains Alice's power, killing her. Furious, the Teen buries the others in mud with magic he didn't know he had, and the sigil breaks.
He's Billy Maximoff, Wanda and Vision's son. When Wanda's Hex collapsed, Billy's soul entered the body of William Kaplan, a teenager dying in a car accident near Westview, and three years later he worked out who he really was. He's walking the Road to find his twin brother Tommy. Lilia, who has lived her whole life out of order, realizes that Rio is Death itself. In a castle full of tarot cards, Lilia solves her own trial, then stays behind and brings the room down on herself and the Salem Seven.
The Road turns out to be a circle. Jen learns that it was Agatha who bound her powers. Agatha unbinds them, and Jen leaves. Agatha helps Billy find Tommy's soul, and Billy places it in the body of a drowning boy. Rio comes to collect, because Billy has upset the balance by living twice, and she demands one of them. Agatha kisses Rio and dies in Billy's place.
In 1750, Agatha's son Nicholas Scratch was born, and Death gave her six years with him before taking him. Agatha spent the centuries after killing witches lured by the promise of a Road she made up from a song she and Nicholas wrote. It was never real, she tells Billy, until his powers made it so. Agatha returns as a ghost, and she and Billy set out together to find Tommy.