Millenia ago, the Dark Elves almost destroyed the Nine Realms with their ultimate weapon known as the Aether. Now the Convergence, when the Nine Realms briefly align, is imminent and the surviving Dark Elves have returned to reclaim the Aether. Meanwhile on Midgard (Earth), astrophysicist Jane Foster discovers portals linking the realms, whereupon she inadvertently becomes the host of the Aether. Thor rescues his lover and brings her to Asgard. Many battles ensue across the Nine Realms as Thor prevents the Elves from invoking the Aether at the Convergence.
I preferred the first Thor film to Dark World. The main characters do get an opportunity to become better rounded, but the film as a whole lacks a spark.