In the space between realities known as the Gap Junction, an alternate version of Doctor Strange races alongside America Chavez toward the legendary Book of Vishanti, a mystical text said to grant the power needed to defeat any threat. Pursuing them is a terrifying demonic entity called the Ribboned Creature, which relentlessly tears through the collapsing pathways between universes. Realizing they cannot escape, Strange makes a desperate decision: he attempts to steal Chavez’s unique power to travel across the Multiverse, even though the process will kill her. Before he can complete the spell, the monster fatally wounds him. Terrified and unable to control her abilities, Chavez accidentally opens a portal to another universe, sending herself and Strange’s corpse crashing into Earth-616.

In New York City, the Earth-616 version of Strange awakens from what he believes is merely a disturbing nightmare. He attends the wedding of his former love, Christine Palmer, where the two quietly reflect on their failed relationship. Christine admits that despite their feelings for one another, Strange’s inability to place anyone above himself doomed them from the beginning. Their conversation is interrupted when a gigantic one-eyed tentacled monster called Gargantos attacks the city in search of Chavez. Strange leaps into action to protect civilians and soon discovers the frightened teenager from his dream. Wong joins the battle, and together the sorcerers defeat Gargantos by impaling its eye. Afterwards, Chavez reveals that demons have been hunting her across the Multiverse because of her rare powers. To prove her story, she brings Strange and Wong to the corpse of the alternate Strange, forcing him to realize his dreams are actually visions into the lives of his variants across parallel realities.

Examining the dead Strange, the sorcerers discover traces of witchcraft connected to the attack. Suspecting chaos magic, Strange seeks help from Wanda Maximoff, believing she may understand the Multiverse better than anyone else. He visits her isolated orchard, but soon discovers the peaceful farm is merely an illusion created by her magic. Wanda reveals that she possesses the corrupted spellbook known as the Darkhold and admits she orchestrated the attacks on Chavez. Consumed by grief over losing her children Billy and Tommy, Wanda plans to use Chavez’s powers to travel freely across the Multiverse and reunite with versions of her sons that exist in other realities. She coldly warns Strange to surrender Chavez before nightfall or face the consequences. Strange refuses, knowing Wanda’s obsession has transformed her into the Scarlet Witch.

At Kamar-Taj, Wong and the Masters of the Mystic Arts prepare for Wanda’s assault by surrounding the fortress with powerful magical defenses. Wanda arrives and unleashes devastating chaos magic upon the stronghold, easily overwhelming the sorcerers. When Strange traps her inside the Mirror Dimension, she horrifyingly escapes by crawling through reflections, proving her power far exceeds anything they anticipated. As the defenses collapse, Chavez accidentally opens another multiversal portal, pulling herself and Strange into a new universe designated Earth-838.

The pair arrive in a futuristic version of New York and search for help at the Sanctum Sanctorum. Instead of finding that universe’s Strange, they meet an alternate Baron Mordo, who informs them that Strange died years earlier defeating Thanos. Pretending to help them, Mordo secretly drugs their tea and imprisons them within a high-security facility overseen by the Illuminati, a council of Earth-838’s greatest heroes. Strange is brought before the group, which includes Captain Carter, Black Bolt, Captain Marvel version Maria Rambeau, Reed Richards, and Professor X. They explain that their own Doctor Strange became corrupted by the Darkhold while searching for ways to defeat Thanos. Although he ultimately located the Book of Vishanti and helped save the universe, his reckless use of forbidden magic triggered an incursion capable of destroying entire realities. Recognizing the danger he had become, their Strange willingly allowed Black Bolt to execute him.

Back on Earth-616, Wanda uses the Darkhold’s forbidden technique known as dreamwalking to possess the body of her Earth-838 counterpart, allowing her consciousness to invade another universe. When the sorcerer Sara destroys the Darkhold in an effort to stop her, Wanda burns her alive in rage. Wong is forced to reveal the existence of Mount Wundagore, an ancient fortress where the original Darkhold spells were carved into stone. Using Wundagore’s power, Wanda dreamwalks once more into Earth-838 and attacks the Illuminati headquarters in pursuit of Chavez. The supposedly unstoppable Illuminati are slaughtered with horrifying ease. Black Bolt accidentally destroys himself after Wanda removes his mouth, Reed Richards is torn apart, Captain Carter is sliced in half by her own shield, and Maria Rambeau is crushed beneath a collapsing statue. Professor X enters Wanda’s mind in an attempt to save the remaining goodness within her, but the Scarlet Witch violently snaps his neck inside the psychic realm, killing him instantly. Meanwhile, Strange escapes captivity after tricking Mordo into freeing him.

With help from the Earth-838 version of Christine Palmer, Strange reunites with Chavez and flees through the collapsing facility into the Gap Junction. There, Strange finally reaches the Book of Vishanti, hoping its power can stop Wanda without killing her. Before he can use it, Wanda destroys the book and seizes control of Chavez. She sends Strange and Christine into another dying universe consumed by incursions while dragging Chavez back to Earth-616 to steal her powers permanently.

In the ruined universe, Strange and Christine encounter a corrupted variant of Strange who has been driven insane by the Darkhold. A third eye has grown on his forehead as a physical mark of the book’s corruption. The sinister Strange warns that every version of himself ultimately causes destruction, but Earth-616 Strange refuses to give up. The two sorcerers engage in a surreal battle using animated musical notes as weapons before Strange kills his corrupted counterpart and takes possession of the Darkhold. Despite knowing the terrible cost, Strange uses the forbidden magic to dreamwalk into the corpse of his dead variant back on Earth-616. As undead Strange rises from the grave, demonic spirits attack him for violating the natural laws of reality. Christine protects him long enough for Strange to bind the spirits to his body like a terrifying cloak.

At Mount Wundagore, Wanda prepares to drain Chavez’s powers while Wong and the undead Strange launch a final assault. Although Wanda initially defeats them, Strange encourages Chavez to stop running from her abilities and trust herself. Empowered by his faith, Chavez masters her powers and opens a portal that sends Wanda to Earth-838. There, Wanda comes face-to-face with Billy and Tommy, but the children recoil from her in terror after witnessing her monstrous behavior. Seeing the fear in their eyes finally shatters Wanda’s corruption. Comforted by her alternate self, Wanda realizes the devastation she has caused across the Multiverse. Filled with remorse, she destroys Mount Wundagore, collapsing the fortress upon herself and erasing every copy of the Darkhold throughout existence.

After the battle, Chavez begins training in the mystic arts at Kamar-Taj while Strange returns to New York. Before parting ways with Christine, Strange confesses that he truly loved her but was too afraid and emotionally distant to make their relationship work. Later, as Strange walks through the city, a painful third eye suddenly opens on his forehead — proof that the Darkhold’s corruption still lingers within him. Soon afterward, he is approached by the mysterious sorceress Clea, who warns that his actions have triggered another incursion threatening reality itself. Opening a portal into the Dark Dimension, Clea asks Strange to help fix the damage, and together they step into the unknown.