The future may be different, with electric lights and telegrams, but the world still needs the Avatar. The Avengers set in the world of Avatar: The Last Airbender. I really can't be more direct than that.

When the Avengers raid a very dubious scientific installation, they come across something they'd never expected to find. Their discovery turns out to be even more surprising than first suspected, and it leads the team - especially Steve and Tony - to some new and very undiscovered territory.