Mia and her friends must reach Mia's dad - without capture by Mr. Sutton, who is attempting to blackmail him - before her dad can help them combat a disease Mr. Sutton has unleashed. Mia is known as the girl who fell down the well when she was four. Her mother has since died, but journalists still enjoy interviewing her. Her interview with Mr. Sutton, however, is not normal. He's trying to use her to reach her dad, a former coworker with whom he is not on good terms.
On Friday of that week, Mia is looking forward to her birthday and that weekend's swim meet when she is sent to the principal's office after she hits a boy who provokes her. As she arrives, she sees a teacher who appears sick through a door. Then an alarm sounds and students are rounded up to the auditorium to be placed in quarantine. Her father calls her to tell her to leave campus and go to the aqueduct close to his work. She doesn't immediately, but wishes she had after they are quarantined in their dorms.
One student tried to run away as they were being quarantined, but is shot and taken to the infirmary. When he returns, he is sick and aging quickly. Mia, Brayden (a newbie she met at a party the previous night), and Jo (a friend of Mia's) break quarantine to check out the teachers offices and find they have all been rapidly aged to death. They come across Mia's father as he is dying, and he is barely recognizable. When Mia sees Sutton there, they run. Apparently it's his men guarding the school. He makes an announcement shortly afterwards asking Mia to come to the front office. He wants her as bait to blackmail her father into allowing him in his facility.
After regrouping at the dorm, the three of them plus Rob (who likes Jo), Odessa (Mia's childhood friend who has somewhat changed in adolescence), and Jimmy (who likes Odessa) escape from campus. In the process, Mia swims through a frozen river, nearly dying and earning herself some numbing, painful frostbite. Fortunately, her friends bring her to barn and warm her up, and Brayden (a remarkably resourceful boy) finds them some food.
They continue towards the aqueduct but are stopped by Sutton's men. Brayden almost seems to be a conspirator at first, and is separated when Mia kicks him. They are locked up in a room together (except Brayden). By now, Jimmy has the virus and looks about thirty. With some obstacles, Brayden finds them and they escape during a fight between Sutton's infected soldiers and the ones who are well. Odessa receives a gunshot wound in the leg.
When they reach the aqueduct, the man who was supposed to bring them to Mia's father has died from the virus. They enter her dad's facility anyway, using information cryptically left in a voicemail on Jo's father's phone. Her father and his two coworkers treat them with rejuvenating spring water that heals everything - the facility's big secret.
It turns out that the six who originally worked there were classmates at Westbrook Academy (the same school Mia and her friends now attend) and discovered the series of caves on a field trip as part of an exclusive elective class. Two didn't make it to town before Sutton began trying to blackmail them. Sutton himself abused the water, turning it to evil purposes, and tried to break promises they'd made about keeping the water secret because he wanted to cure his wife's cancer. So they'd kicked him out. Now he'd released the virus he created from the water in an attempt to make them let him back in. The water itself returns once every 17 years for about 10 days; the last time was on the day of Mia's birth. It is now the morning of her birthday.
Sutton does break in - with help from Brayden. Brayden's parents are sick, and he wants Sutton to help him cure them with the remaining vial from the Cave's previous harvest. Mia feels betrayed. On her way back to the recreation room (where she and Brayden should have been), she meets Rob and Jo, who had come looking for them. The three find the room with the well (which had thus been refused to them). They block the elevator so Sutton can't come down. The water comes, and Mia leads the way down. She is sure now that is what they need to do: seek the source.
Best part of story, including ending:
There's too much focus on the teenagers' romantic feelings and behavior. There's too much drugs and alcohol, especially at the beginning. The characters use more language than they really need to. For about the first two thirds of the book, it doesn't move quickly enough.
Best scene in story:
I like the very last scene, where Mia and her friends dive into the well to find the source. There is a parallel here with her fall into the well at age four. Though it's not a single scene, I also liked the section where her dad explains about the cave, how it was discovered, and about the work he and his classmates have done since. It's written in 3rd person instead of 1st person (most of the book is written in 1st person from Mia's perspective), and the tone is a little different, reflecting the flashback and explanatory nature of the section. As a result, the story also starts moving a little more quickly at this point.
Opinion about the main character:
I like Mia's dedication to her swimming, and the strength it gives her to face her fears. I don't like how quick she is to go gaga over Brayden, or at least to be slower and more reserved in responding to his advances.