So the world is ending, and some scientists build a refuge underground and they set it up to last for 200 years. The city is powered by a huge generator and the mayor of this city is in charge of the exit instructions to return to the surface after 200 years. But one of the mayors dies and the box is lost. So now time has run out and the City of Ember is dealing with a dwindling food supply and blackouts that last longer and longer. No one knows that there was a timeline to how long citizens remain in the city and so they all continue on, living an increasingly unsecure life.
But there are some who question the rules and laws of their city. Such citizens are Dune and Lina, two teenagers who are friends and both going through the same rite of passage to the youth of Ember. It's called assignment day and this is where the mayor comes along with a bag full of jobs and the teens randomly select their lot in life. Lina picks "Pipeworks" and Dune picks "messenger." They quickly switch because Dune wants a chance to get to the generator (thinking he can fix it or discover the cause of the blackouts) and Lina yearns to be a messenger.
So things start happening in Ember that Lina and Dune notice and they become embroiled in a plot to find a way out of Ember. Which is treason. So they're on the run while time is running out.
City of Ember is based on a young adult novel, but as a movie, I see no problem with adults enjoying it. I know I did! I love these kinds of movies (think of the Matrix LOL) that have conspiracy theories and underground worlds. I'm such a geek. But yeah, if you want to get sucked into a different world - check out City of Ember.