Two hundreds years after being established underground as the last bastion of civilization following an apocalypse above ground, the city of Ember is facing various problems. While a food shortage is on everyone's minds, the city's failing generator -- that supplies all electricity and light to the citizens -- is apparently starting to fail, noted by increasingly prolonged blackouts.
Mayor Cole (BILL MURRAY) tries to assure everyone that all will be fine, but for young Doon Harrow (HARRY TREADAWAY), Assignment Day couldn't come soon enough. That's when the young people of Ember learn of their upcoming vocation -- decided by the literal luck of the draw -- and Doon, son of inventor Lorris (TIM ROBBINS), desperately wants to be on the generator team in hopes of fixing it.
Unfortunately, his draw is that of city messenger, an assignment he willingly exchanges with his friend Lina Mayfleet (SOAIRSE RONAN) who drew a job working in the city's aging water pipe section. While she lives with her increasingly senile Granny (LIZ SMITH), she's essentially raising her younger sister, Poppy (AMY QUINN & CATHERINE QUINN), and longs to hear stories of her late father from greenhouse worker Clary (MARIANNE JEAN-BAPTISTE).
As she runs around the city delivering messages, including from empty stockroom worker Looper (MACKENZIE CROOK) to the Mayor, Doon begins working with pipe works veteran Sul (MARTIN LANDAU). Of course, he really wants to get inside the generator room, but while exploring for a way to get in, he makes discoveries that pique his interest in what lies outside the city in the forbidden "unknown region."
Lina, who's discovered a tattered and partially incomplete exit plan left long ago by the original builders, shares that interest, particularly due to her father also doing the same in the past. Accordingly, they set out to find a way out, but must deal with Mayor Cole's lieutenant, Barton Snode (TOBY JONES), who's determined to make sure they don't succeed.