The narrative unfolds over a single day as four family members enjoy their vacation on a secluded island, shortly after Karin, played by Harriet Andersson, is discharged from an asylum where she was receiving treatment for her schizophrenia. Her husband, Max von Sydow, known as Martin, a dedicated doctor, informs her father, Gunnar Björnstrand, that Karin's condition is nearly impossible to cure. In the meantime, Minus, played by Lars Passgård, Karin's 17-year-old brother, expresses to her his yearning for a genuine conversation with their father and laments feeling unloved. David, a novelist grappling with severe writer’s block after a prolonged trip abroad, reaffirms his plan to leave again shortly, despite having previously promised to stay.
In an attempt to bring the family together, they perform a play written by Minus for David. Although David pretends to appreciate the performance, he secretly feels insulted, interpreting the play as a critique of his character.
That evening, after resisting Martin's sexual advances, Karin is drawn by the haunting sound of a foghorn to the attic, where she faints from an overwhelming episode, hearing disembodied voices through the crumbling wallpaper. Curious, she rummages through David's belongings and finds his journal, which chillingly describes her illness as incurable and reveals his cold wish to document her decline. The next day, while fishing, Martin accuses David of exploiting Karin for his own literary gain and of being self-centered and insincere. David dodges the accusations but concedes that Martin’s criticisms hold truth. He confesses to having contemplated suicide by driving off a cliff, only to be thwarted by a malfunctioning vehicle—a near-death experience that helped him realize his love for Karin, Minus, and Martin, thus kindling his hope.
Karin confesses to Minus about her troubling episodes, sharing her anticipation of God revealing Himself through the attic's wallpaper. As Minus grapples with his own sexual frustrations, a teasing interaction ensues between him and Karin. In a moment of chaotic vulnerability as a storm brews, Karin seeks refuge in a wrecked ship, where an inappropriate encounter occurs between her and Minus.
After Minus divulges the incident, Martin urgently calls for help. Karin requests a private conversation with her father and admits her wrongdoings towards Martin and Minus, claiming an unseen voice compelled her actions. She expresses a desire to remain in the hospital, as she finds the conflict between her two realities unbearable. During their departure preparations for the hospital, Karin rushes to the attic, where Martin and David witness her frantic behavior. She insists that God is about to emerge from a closet, yearning for one final moment of enjoyment, and becomes transfixed by a spider crawling from a crack in the wall. Suddenly, the noise of a helicopter ambulance disrupts the tension, causing her to freeze in terror, and despite her initial eagerness, she recoils in panic, ultimately succumbing to a frenzied state.
As she is sedated, Karin recounts a surreal vision of God—a spider with a menacing visage that attempted to invade her being. She boldly declares, “I have seen God.”
In the final moments, as Karin and Martin depart in the helicopter, Minus confides in his father about the fear he felt during the encounter with Karin, questioning the fragility of reality. In reply, David reassures him that survival hinges on having something to cling to; he shares his own source of hope—love. Their discussion touches on love’s potential as a stabilizing force, offering comfort to the notion that their love could aid in supporting Karin's complicated journey. Grateful for this newfound connection, Minus joyfully exclaims, “Papa spoke to me.”
