We return to the world of Asgard and face a new threat to the universe in the sequel to Thor.
Long, long ago, the universe was threatened by a race called the Dark Elves. The Elves, and their leader Maleketh planned to use the power of their great weapon, the Aether, to return the universe to darkness, as it had been when they were born, but they were defeated by Odin’s father Bor and believed to be extinct. The Aether was hidden away, presumably lost forever.
In the present, in London, Jane Foster has been studying weird science readings, and discovers a place with strange portals. She gets sucked into one of these portals and transported to a distant world where she encounters the Aether, and it is absorbed into her, using her as a host. This event causes Maleketh and his remaining dark elves to wake from their centuries long slumber to find her.
Meanwhile, with the destruction of the bifrost during Loki’s original betrayal of Asgard, and due to some of his dealings after he left, the nine realms has broken out into near chaos. Now that the bifrost is finally repaired, Thor and the Thorettes have been busy quelling the chaos and taking down groups of roving marauders, restoring peace. Loki, captured after the Battle of New York, is held guilty by Odin and thrown into prison. Despite being super busy, Thor is still obsessed thinking about Jane, and asks Heimdal about her frequently. When Heimdal realizes he cannot see her, Thor rushes to Earth to check on her, just as she returns from her trip through the portal, unaware of what has happened to her.
Jane is nearly arrested for trespassing, but when the police attempt to grab her they are thrown back by powerful energies, and she starts to collapse. Thor immediately grabs her and takes her back to Asgard to have her checked out. Despite not being able to explain how it is possible, Odin and Thor determine that she has the Aether, and that it will eventually overcome and kill her. Before they can work out how to remove it, a riot breaks out in the prison, leading Thor to investigate and help. One of Maleketh’s lieutenants managed to sneak into the prisons disguised as a marauder, and used and ancient elven weapon to turn himself into the nearly unstoppable Kurse. With his new power Kurse broke out of the prison and let loose most of the other prisoners as a distraction.
Maleketh and the dark elves show up to take the Aether for themselves, causing serious damage to the castle. Kurse makes his way to the shield generator to leave the city defenseless so Maleketh can infiltrate the throne room. With Thor fighting the marauders and Odin fighting Elves, Odin’s wife Freja watches over Jane. Maleketh, sensing the Aether, approaches them and kills Freja, but is heavily injured by a returning Thor, forcing the Elves to leave for the time being.
Distraught, the Asguardians hold a funeral ceremony for their fallen, and prepare for continued war. Odin orders the bifrost closed and the city locked down, planning to mount a defense when the elves return, but Thor has other plans. He intends to take Jane away from the city, luring Maleketh somewhere else and allowing him to remove the Aether from her, making it vulnerable and able to be destroyed, no matter what his father orders. To aid him in this he busts out Loki, the only person that knows the hidden backways between worlds. Loki agrees to help, looking for vengeance for his mother.
With the help of the Thorettes, Thor, Loki, and Jane manage to steal one of the abandoned dark elf ships to create a distraction allowing them to make their way to the dark elf planet. Their sudden appearance there calls off a second attack on Asgard, and immediately attracts Maleketh’s attention. Loki pretends to betray Thor, leading Maleketh to get close enough to take the Aether from Jane and giving Thor a chance to destroy it, but unfortunately it does not work, and Maleketh absorbs the Aether himself, leaving Kurse and his elves to finish the Asguardians off. Loki manages to kill Kurse with one of his own black hole grenades, but is terminally wounded in the process. With an incoming dust storm, Thor and Jane take cover in a cave and try to decide what to do next.
An unexpected phone call from across the galaxy leads them to discover that the cave is connected to the portals Jane discovered in London, allowing them to return to Earth. Thor explains that the barriers between worlds are growing thin due to an event called the convergence, and they realize that Maleketh plans to use the Aether and the convergence to destroy all the worlds at once. With some help from Doctor Selvig they are able to determine that the focal point of the convergence will be in Greenwich, and that is where Maleketh will be. They modify some of Selvig’s anomaly detection devices so that they can create portals of their own for distraction and set out to keep Maleketh busy for the length of the convergence’s peak.
Thor and Maleketh fight across multiple worlds, hopping from portal to portal as Thor tries to keep him busy, but Maleketh manages to lose him at just the right moment, and calls upon the Aether to make a giant storm, keeping Thor away from him and spreading out through the portals of the convergence to enact his plan to destroy everything. Thor is able to force his way through at the last moment and use on of the portal devices to send Maleketh back to his home planet, dropping a large portion of his ship on him in the process.
With the universe restored, Thor goes to his father and asks not to be made King, choosing instead to keep time free for protecting the Earth and spending time with Jane. Odin is sad, but understanding of his son’s wishes, granting him his leave and choosing to stay in power. Only it isn’t Odin at all, but Loki, having faked his death, and secretly taken Odin’s place in the chaos.
The Aether, recovered from Maleketh’s body, is taken to the Collector, with the Thorettes citing that they already have the Tesseract in their vaults, and it isn’t safe to keep two infinity stones in one place.
TLDR: Thor returns to Earth to save Jane Foster who accidentally finds a powerful ancient weapon, wakes up a bunch of Dark Elves, and nearly gets the universe destroyed.