Five thousand years before the present day, the universe experiences a rare cosmic alignment known as the Convergence, a phenomenon that weakens the boundaries between the Nine Realms and opens unstable portals connecting them. During this event, the Dark Elves, an ancient and malevolent race from Svartalfheim, attempt to use a terrible weapon called the Aether to plunge existence back into primordial darkness. The Aether is a mysterious red, fluid-like force later revealed to be one of the six Infinity Stones, artifacts older than the universe itself. Under the command of their ruthless leader Malekith, the Dark Elves come close to erasing all life and reshaping the realms according to their twisted vision. They are stopped by Bor, King of Asgard and father of Odin, who leads the Asgardian armies into battle. Bor seizes the Aether before Malekith can complete his plan, and the Dark Elves are seemingly annihilated. Unable to destroy the weapon, Bor’s forces hide it where no one can find it. Unknown to them, Malekith, his lieutenant Algrim, and a handful of survivors escape into suspended animation, waiting for the day the Convergence returns.

In the aftermath of the Battle of New York, Loki is brought back to Asgard in chains and sentenced by Odin to life imprisonment for his crimes against Earth. Thor, meanwhile, spends the next two years restoring peace across the Nine Realms, which have been thrown into chaos after Loki’s actions destabilized them. One of Thor’s final campaigns takes place on Vanaheim, where he and his allies Sif, Fandral, Volstagg, and Hogun defeat a marauding army. Victorious, Thor returns to Asgard as a celebrated hero, but despite the glory, his thoughts remain fixed on Jane Foster. He repeatedly asks Heimdall to watch over her on Midgard. Odin disapproves of Thor’s attachment to a mortal woman, warning that her short human lifespan makes such love painful and impractical.

On Earth, Jane Foster has relocated to London, where she works with Darcy Lewis and intern Ian Boothby investigating strange gravitational anomalies. Having heard nothing from Thor since New York, Jane reluctantly goes on a date with Richard Madison, but it is interrupted when Darcy discovers bizarre readings from Jane’s equipment. The trio follows the signals to an abandoned warehouse where reality itself has become unstable. Gravity shifts unpredictably, objects vanish into thin air, and portals open into distant worlds. Jane is pulled through one such portal and transported to a hidden chamber on another world. There she discovers a strange vault containing the long-lost Aether. When she touches it, the red substance surges into her body and merges with her. She later reappears in London, weakened and disoriented. Police officers responding to trespassing complaints attempt to arrest her, but the Aether violently repels them. At that same moment, Thor arrives, having come to Earth after Heimdall mysteriously lost sight of Jane. After an emotional reunion—punctuated by Jane slapping him for abandoning her—Thor realizes she is in grave danger and brings her to Asgard for help.

In Asgard, Jane is examined by healers, but none can remove the force inside her. Odin recognizes the Aether and reveals its history. As he searches for a solution, Queen Frigga comforts and protects Jane, showing her compassion that Odin lacks. Loki, imprisoned in his cell, remains bitter and isolated, though Frigga continues trying to reach him. Jane’s awakening of the Aether alerts Malekith, causing him and the surviving Dark Elves to rise from their long slumber. They launch an assault on Asgard using cloaked warships. Algrim allows himself to be captured and transforms into Kurse, a monstrous super-soldier. During the invasion, Frigga uses illusions to hide Jane and confronts Malekith herself. Though she fights bravely, Kurse intervenes and fatally wounds her. Thor arrives too late to save her, but in fury he strikes Malekith with lightning, scarring his face and forcing the invaders to retreat. Frigga’s death devastates the royal family. Odin becomes consumed by vengeance, while Thor concludes that Malekith will return for Jane and the Aether.

Defying Odin, Thor resolves to take the fight to Malekith himself. He frees Loki from prison, believing Loki’s knowledge of secret paths between realms can help them escape Asgard. Joined by Jane, they flee using a stolen Dark Elf ship and travel to the ruined wasteland of Svartalfheim. There Malekith confronts them. Loki appears to betray Thor, severing his hand and delivering Jane to the Dark Elf leader, but it is revealed to be an illusion. Malekith extracts the Aether from Jane and absorbs it into himself, becoming vastly more powerful. Thor battles Kurse while Malekith departs to exploit the Convergence. Loki returns to aid Thor, and together they defeat Kurse when Loki triggers one of the Dark Elf’s own implosion grenades. Mortally wounded in the struggle, Loki seemingly dies in Thor’s arms after apologizing for his betrayals.

Thor and Jane return to Earth through a portal and reunite with Darcy and Erik Selvig, whose mind had been damaged by Loki’s control during the Chitauri invasion. Selvig has spent months studying the coming Convergence and has built devices capable of stabilizing or redirecting portals. He determines that Malekith’s target is Greenwich, where the alignment will peak. There, portals open across London and Malekith begins releasing the Aether into the fractures between worlds. Thor battles him across shifting portals that hurl them between realms. Using Selvig’s devices, Thor realizes he cannot destroy the Aether, but he can kill its host. He impales Malekith with the teleportation devices, sending him back to Svartalfheim just as his collapsing warship crashes down and crushes him to death. The portals close, ending the threat.

Afterward, Thor returns to Asgard. Odin praises his courage and declares he would make a wise king, but Thor refuses the throne, choosing instead to live by his own path and return to Jane on Earth. Once Thor leaves, “Odin” transforms into Loki, revealing that he survived and secretly seized the throne of Asgard. In a separate matter, Sif and Volstagg deliver the Aether to Taneleer Tivan, the Collector, explaining that with the Tesseract already stored in Asgard, keeping two Infinity Stones together would be dangerous. Tivan smiles greedily and declares, “One down, five to go.” Back on Earth, Thor reunites happily with Jane, unaware that Loki now rules Asgard in disguise.