NOTE: This spoiler sent in by William who says... "A fantastic movie. The set design was just awesome and the acting was great. See it, see it now!"
The film starts off with a scientist in some kind of research facility/bunker, walking into a room full of other scientists (later referred to as “the builders”). It is evident that there is a battle going on outside as explosions are heard and the facility seems to be crumbling with each explosion. They are sitting around a table with an open metal case on it. The builders discuss when the box should be opened and the “lead” builder says that 200 years should be enough for their descendants to forget all that has happened and will be ready to start over. They place a folded piece of paper with instructions on how to leave the city, as well as a plastic key card into the box and seal it, setting it for 200 years.
A monologue begins and describes the box being secretly transferred from one mayor (Bill Murray) of Ember to the next. However, the 7th mayor is murdered (most likely shot) and the box was lost (since no-one but the mayor knows about it). The box is put away in a closet by family and forgotten, the clock still ticking down until it finally reached zero and opened, but with no one around to notice.
The opening credits ends and we see a ceremony in a run-down school auditorium, Assignment Day, when children (in their early to late teens) are assigned the job they will do for the rest of their lives - by randomly drawing it out of a bag. Here our main characters are introduced: Doon Harrow (Harry Treadaway), the son of an electrician/engineer, wants desperately to become and electrician so he can help fix the generators that power the city of Ember. The generators have been stalling more and more, and the blackouts have become longer and longer. It seems that the city was not meant to function much more than 200 years and is starting to fall apart. Lina Mayfleet (Saoirse Ronan), the other main character sees that she is late, and runs through the streets to get to Assignment ceremony. She is able to sneak in just in time. Dune gets assigned as a messenger, while Lina is assigned to work in the city’s pipe-works. Neither is happy with their assignment and decides to trade. Lina gets her dream of being a messenger (she loves running and exploring the city) and Dune gets to work somewhere where he can be close to the generators and maybe get a chance to fix thing.
It is the first day of their jobs and both Lina and Doon go to their respective work places. Lina works for a post-office of sorts, where messengers run though the town and are paid to deliver verbal messages. Doon is put to work “fixing” the pipes under the city, which involve pasting bandages on to leaking pipes as there hasn’t been replacement pipes and parts for many decades. He is taught by Sol who seems extremely old and has been working the pipes for decades, and his job seems to be fixing the pipes and watching the main pressure gauge in the pipe-works control hub. Doon is given a helmet and told to etch his name into it in case he gets killed, so someone can identify who died, the last person to wear that helmet had died of drowning, his name was Mayfleet.
On his way home, Doon finds a wounded moth (the size of a dog!) and stitches its wing with his shoelaces.
The power goes out once again and people are frightened. This time, the power is out for more than 7 minutes, twice as long as usual. Doon and Lina find each other in the darkness and discuss the fate of Ember. The lights come back on and Lina goes back home to her ailing grandmother and little sister, Poppy (who has a propensity to chew things). Lina calms Poppy by playing old answering machine with their father’s voice (important later). Back at the generator plant, the engineers and electricians are freaking out, the generator is on fire and things are falling apart. Doon tries to help but there is really nothing he can do. Word from the engineers is that things are going to get worse (i.e. total blackout, forever).
At home, Lina’s Grandmother is searching frantically for something, but does not remember what. Lina is looking for Poppy, and finds her in the closet, chewing on something from an opened box. She comes out with Poppy and the box and her grandmother recognizes the box, but still does not know what it’s for, just that it is important.
Lina then goes back out to deliver messages, as well as visit her friend who works in a greenhouse at the edge of the city. Soon after she arrives, a scruffy, insane-looking man staggers in from the dark, clutching what looks like a giant-sized beetle mandible (jaw). The man is promptly arrested as it is illegal to leave the city. Lina questions why it is illegal to leave but the issue is tossed aside by her friend as she believes that it is more important to do one’s part and not worry about everything else.
While Lina is back at home, putting Poppy to bed, another blackout occurs and she grabs Poppy and runs to her grandmother’s room. When the lights come back on, Lina’s grandmother has past away. Lina and her sister are adopted by a foster parent (a woman who has a lot of faith that the builders will return to save them, and that everything will be ok).
Later, after work, Lina sees her friend Lizzie (Lucinda Dryzek) who works at supply-storage, which distributes goods to the people. She is carrying a large amount of canned goods and it turns out that her boyfriend, Looper is hoarding supplies for himself as well as the mayor despite dwindling rations for everyone else.
In a conversation with Dune, Lina reveals that her last name is Mayfleet, which reminds Doon of the name etched in the helmet he was given. Lina goes to her friend at the greenhouse, who use to know her father. Lina breaks down in tears as she misses her father. At this point her friend reveals a hidden cave behind her greenhouse, where Lina’s father had created a plan to leave ember. Lina speaks to Doon about her father’s attempt while putting Poppy to bed when Doon recognizes a voice on the answering machine tape; it’s his father, Loris Harrow (Tim Robbins). It turns out that many years ago, Loris and Lina’s father had attempted an escape plan, but it failed and Lina’s father drowned. Since then, Loris has given up on the plan.
Lina decides that the box may hold the key to leaving Ember, however, a good portion has been damaged by Poppy (chewed). She pastes the parts together and is able to derive some information, along with finding the key card in the box. Lina and Doon decide to the underground pipeworks some more to see if there are more clues left behind by the builders.
Lina and Dune go to investigate and come upon a locked steel door. They hear Looper coming around the bend, but there is a loud nose and a HUGE shadow runs by, chasing Looper. Lina and Dune decide to leave and out of the shadow, a gigantic mole comes charging at them, teeth gnashing about. The two escape through the hole in the ceiling Dune had discovered earlier, and run through the store room, chased by the giant carnivorous mole.
They return to the locked door and Dune picks the lock and finds a chamber completely filled with food, along with the mayor passed out on a couch, surrounded by empty food cans. It turns out that the mayor has struck a deal with Looper to hoard food from the public stock-piles and that there really isn’t a food shortage, just the mayor being greedy.
Lina and Dune leave and the part ways. Dune goes back to the pipe-works to see if he can help, and Lina goes to the mayor’s office in hopes to getting someone to listen to her claims and bring the mayor to justice. However, it turns out that the person she spoke to was on the mayor’s side and she gets hauled into the Mayor’s office. While there, the Mayor is trying to decide what to do with Lina but then he spots the box Lina is carrying. He sends his goods to grab the box but a blackout occurs and Lina is able to escape through the window, along with the plastic card the mayor has on a chain. The mayor cannot risk Lina exposing him, and sends his goons (as well as the police) after her, one of his goons note that she is friends with Doon Harrow, and men are sent after him as well.
At this point, the two are now wanted by the police. Lina escapes to her friend’s greenhouse at the edge of the city. There she is hidden in the secret compartment her dad had made. At the same time, Doon is also pursued by the police but makes it home, hiding in the attic. His dad sees the cops coming and stalls them the best he can, pretending to be a crazy person, yelling senseless phrases, which are in fact clues on how to escape Ember (as he and Lina’s father had tried to escape years earlier, but failed).
Dune escapes to the greenhouses to meet with Lina, while the mayor’s goons are ransacking the place. The head goon thinks he sees someone escaping and chases after. As the mayor’s lackys and police leave, Lina helps Doon into the secret chamber. Inside the chamber/cave, Doon recognizes the car-like contraption as something his father made (it looks like a bladed deathtrap with wheels). The two get on and pedal their way through a series of tunnels and to another site at the edge of the city. At this point, the two stop to look through the instructions. It seems that the two cards (one from the box and one from the mayor) connect to form a key card. The instructions indicate that the exit is somewhere in the power generator plant. Lina decides to find her sister and take her with them, so she and Doon sneak back into the city and are able to get to Poppy while everyone is distracted by the Day of Singing Ceremonies.
Together again, the three make their way into the power plant. From the instructions, Lina figures out that the power plant was designed to help people leave the city of Ember when the time was right. Activating the control panel, the “lockers” in the power plant turn out to be little boats that are moved towards the underground river the power plant is based on. The first boat is deposited into the river, but is soon destroyed by the water wheel/turbine at the end of the river. Doon surmises that there is another step that needs to be taken and consults the instructions. The instructions say that the turbine needs to be lowered using the controls in the tower at the top of the generator so the three head up there. However, upon activation, one turbine (the one on the exit) jams, and the area begins to flood.
Sol, at his usual post sees the pressure rise, and finally does the job he was trained for, to lower the turbine. He runs out onto the turbine but does not have the right tool to fix it, however, Doon arrives on the scene and hands him the tool his father gave him. Sol pries the gears together and the turbine continues to run, but in order for them to get out, Sol has to stay to keep the turbine running.
While this is happening, the city is in a state of panic power is going on and off, and the ground is shaking. The mayor makes a run for his safe-house (the room filled with food). He makes it there, but Looper wants in too, but the mayor refuses to let him in, and locks Looper out. Just when he thinks he is safe in his room, he sees a very hungry, very large mole come at him…
Back at the power plant, Doon, Lina, and Poppy get into the next boat and float out towards the exit. What ensues is a roller-coaster ride of sorts. The boat follows a series of underground river-ways, down a few waterfalls and almost capsize a few times. The boat finally reaches the end of the river and they see the way out. The exit is at the end of a very long and high staircase and after much stair-climbing, they make it out to the exit.
They finally leave the tunnels but to their dismay, it is completely dark outside, just like the rumours they had heard. Having lost much hope, the three camps out outside to contemplate what to do next, however, soon, the sun rises to reveal a beautiful wilderness filled with wildlife.
Doon sees the giant moth he had cared for earlier (he recognized the shoe-lace stitches) and figures out that it must have found a way out of ember. They follow it and find a crack in the ground and see that they are directly above the city of Ember, however, they are so high up (or Ember is so deep) that the citizens below cannot see them. Lina and Doon decide to send a message to those in the city by wrapping a note around a rock and drop it.
By shear luck, the person who picks up the rock is Loris, Doon’s father, who realizes that his son had made it out.