Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Exposition (Initial Situation)
The Builders' Intentions; The Job Lottery
From page one we learn that Ember is a carefully designed city, made by the Builders to protect the people in it for a certain span of time. When we jump into the novel's main plot, it's Assignment Day, which the kids in Class 8 are kinda excited about and kinda scared about. We're calling this the initial situation because we learn the background with Ember, and we meet and learn all about Lina and Doon, our main characters
Rising Action (Conflict, Complication)
Not Everything Is As It Seems
Lina runs around happily being a messenger; Doon plods along in the Pipeworks trying to fix stuff. And yet not everything is hunky-dory. Both kids realize that something is truly wrong in Ember. The power is failing, supplies are running out, the ghosts of Christmas Past are… wait, different story.
When Lina finds a message containing instructions from the Builders, and when Doon finds a secret room in the Pipeworks, they start to figure out that urgent action is needed to save their community. This, of course, makes their lives pretty complicated because brings them into conflict with the mayor, who benefits from keeping his people in the dark about the situation.
Climax (Crisis, Turning Point)
What is this, Waterworld?
After a mad dash to figure out the cryptic instructions and locate the point of exit from Ember, Lina is captured by the mayor's guards for treason and Doon is helpless to save her. She finally gets away, and together the kids (along with Lina's baby sister Poppy) maneuver one of the Builders' escape boats into the river.
Cue exciting trip down a raging waterway. It's all new and scary and, of course, very wet. Being pursued by the authorities is freaky and tense, hence the main crisis of the plot, and the plot turns in a new direction once the kids realized that they can and should escape by boat.
Falling Action
The First Sunrise
The kids emerge into the world above, see the moon and stars, and witness their first sunrise (which is totally amazerrific). They realize that they've come from this world, and were meant to return to it. They also read a journal from the time of the Builders, helping them put together more puzzle pieces.
Since most of the previous action has taken place over the last few days, the chance to catch their breath in a peaceful, scenic nature-y spot is pretty much the best falling action they can hope for.
Resolution (Denouement)
We're Here, You're Next
Doon and Lina find a crack in the earth and manage to get a glimpse of Ember, which has been underground this whole time (oh, so that explains it). They throw a message down into the city, explaining the mayor's treachery and how to access the escape boats on the river. Mrs. Murdo finds the message—and because we know she's super-competent, we're thinking she'll act on it ASAP. We're filled with a sense of hope that a lot of the problems in Ember (abuse of power, lack of electricity, a dearth of supplies to sustain life) will be resolved once this information reaches everyone and they hatch a plan to come out of the darkness and into the light.