Full story
Thousands of years ago, Odin's father, Bor, defeated Malekith, leader of the Dark Elves, who wanted to use a weapon called the Aether to plunge the universe back into darkness. Bor hid the Aether where no one could reach it. Malekith escaped with his lieutenant Algrim and a few soldiers and went into a deep sleep.
In the present, Loki is locked up in Asgard for his crimes in New York. With the Bifrost rebuilt, Thor has spent two years restoring order across the Nine Realms with Sif and the Warriors Three. The Convergence is coming, a rare alignment of the realms that opens passages between them. In London, Jane Foster investigates one of these anomalies with Darcy, slips through a portal and absorbs the Aether. The weapon waking up wakes Malekith too.
Thor comes to Earth and takes Jane to Asgard. Odin explains that the Aether is ancient, dangerous and slowly killing her. Malekith attacks Asgard to get her. Algrim becomes a Kursed, a nearly unstoppable warrior, and frees the prisoners. Frigga, mother of Thor and Loki, protects Jane and is killed by Malekith. Her funeral, with burning boats, is one of the most beautiful scenes in the film. Odin wants to wait for the next attack even if it costs lives. Thor disagrees.
To sneak Jane out of Asgard, Thor turns to the one person who knows the secret paths: Loki. The brothers bicker, trade jabs and, for a moment, connect over their mother's death. On Svartalfheim, Loki pretends to betray Thor and hands Jane to Malekith, who pulls the Aether out of her. It's a trick, but Thor fails to destroy the weapon, and it merges with Malekith. In the fight, Loki kills Algrim and is mortally wounded. He dies in his brother's arms, or seems to.
Thor and Jane return to London, where a rattled Erik Selvig has built gadgets to deal with the Convergence. Malekith shows up in Greenwich to release the Aether at the center of the alignment. The final fight jumps between realms through portals, with the hammer chasing Thor from world to world. Jane and her team use the gadgets to send Malekith back to Svartalfheim, where his own ship falls on him and crushes him. Thor turns down the throne of Asgard and chooses to stay with Jane. The final shot reveals that Odin on the throne is really Loki in disguise. In the mid-credits scene, Sif and Volstagg hand the Aether to the Collector, since keeping two Infinity Stones together in Asgard would be unsafe. He dreams of the rest.