A drought known as the Tap-Out leaves California with a severely decreased water supply. Then one day, the taps dry out completely. Teenage Alyssa, her 10-year-old brother, Garrett, and Uncle Basil go to Costco to buy bottled water and notice the tense, combative attitudes of everyone in line. The cases of water are already gone, but Alyssa comes up with the clever idea to buy a cart full of bagged ice to melt for water. A man attempts to take it from them.
Alyssa’s next-door neighbor Kelton McCracken is a survivalist geek. His parents are prepared for the apocalypse, complete with a well-guarded home, a stockpile of water and a backup generator for power. The leader of the homeowners’ association comes to the McCrackens’ house with an expensive present, hoping that in return they’ll share their resources with others, but Kelton’s father isn’t sympathetic to people who are unprepared for a crisis.
Day by day, the situation worsens. Alyssa’s has a friend named Sofia whose entire family packs their belongings and leaves California. Then Uncle Basil leaves to stay with his ex-girlfriend so he won’t use up any more of Alyssa’s family’s water. On the news, the first death from dehydration is recorded, and incidents of people attempting to steal water from a hospital are mentioned.
On the third day after the taps run dry, Kelton grows concerned. Even though his family is better prepared than anyone else, it still seems like they’re not ready for the magnitude of this crisis. His thoughts also turn toward Alyssa, on whom he has a long-term crush.
Tragedy strikes at Alyssa’s house when Garrett accidentally knocks bleach into the bathtub full of drinkable water they had been hoarding. Afraid of his parents’ disappointment, Garrett runs away from home on his bike. Alyssa’s parents go to look for him. Alyssa borrows a bike from Kelton, who rides along with her to help with the search. They find Garrett in an abandoned house where one of his friends once lived. The friend had owned a giant fish tank, which has now been smashed by thieves trying to get the water.
At Kelton’s house, there’s tension between his parents because his mother wants to share their stockpiled water with needy neighbors, but his father thinks they will only have enough to keep themselves alive.
The power in the neighborhood turns off completely, and with Alyssa’s parents gone to the beach to get water from a desalination station, Kelton offers to stay overnight with Alyssa and Garrett to help them feel more secure. The next day, all three of them travel on their bikes to find Alyssa’s parents. When they arrive at the beach, they find a disaster zone with no people. The tear gas cannisters littered on the ground indicate that there was a riot. The desalination machines are broken on the beach, apparently torn to shreds by rioting people.
As the kids leave the scene, Alyssa sees some teenagers beating up an older man who won’t give them the keys to his BMW, where he has a hidden water bottle. Alyssa intervenes and is attacked instead. When the leader of the teenagers grabs Alyssa, Kelton produces a handgun and threatens him. Kelton can’t bring himself to use the gun.
When the leader knocks it out of his hand, a beautiful teenage girl picks up the gun and aims it at the leader, threatening to kill him unless Alyssa gives her the keys to the car with the water in it. The girl’s name is Jacqui, and she agrees to drive Alyssa, Garrett and Kelton home in return for Kelton giving her medicine for an infected cut on her arm.
Mr. McCracken isn’t happy about Kelton bringing home three other people, but he allows them to stay. Alyssa steals some water from the McCrackens and sneaks out to give the water to the families at a neighborhood meeting. However, there are 17 families represented at the meeting and only 12 bottles of water.
Soon people begin to argue angrily about the division of resources. Alyssa slips back to the McCrackens’ house with a newfound realization about how quickly human nature turns ugly during times of scarcity. On the news, martial law is declared in most of California.
In the middle of the night, the McCrackens hear someone at the door. When the door opens, Mr. McCracken shoots the intruder before realizing that the man he just shot is his oldest son, Brady. At the same time, the desperate neighbors breach the McCrackens’ outer fence, coming to steal their water.
Alyssa knocks Kelton unconscious before he starts shooting his neighbors in a fit of anger. The McCrackens are too busy mourning Brady to leave their house, so Jacqui drives away in the BMW, carrying just Alyssa, Garrett and Kelton. To get to the McCrackens’ secret bugout hidden in the woods, Kelton says they will need a vehicle with a high chassis, the kind of off-road vehicle that Alyssa’s Uncle Basil owns.
They drive to a wealthy suburb to find Uncle Basil. They discover that he and his ex-girlfriend have been drinking contaminated water from the suburb’s well. Uncle Basil now has dysentery. Uncle Basil tells them he gave the truck to a wealthy teenager named Henry in exchange for bottled water.
When they force their way into Henry’s house to get the truck’s keys, he produces a gun. Kelton rushes toward him and twists his arm out of the socket. It turns out the gun wasn’t real, and Kelton pops Henry’s arm back into place. Henry offers Alyssa a deal: He will give them her uncle’s truck, if they take him with them. He offers to throw in a case of bottled water, not revealing that there’s not actually any water inside the heavy box.
The five kids drive through dry riverbeds to reach the Angeles National Forest where the McCrackens’ bugout is. They stop to siphon gas from some abandoned cars near the river and accept help from a camp of kind people managed by a woman named Charity. Charity finds water for her group by harvesting it from the window-washing fluid tanks of abandoned cars. The kids rest and leave the camp the next day, listening to radio reports about increasingly frequent forest fires in the area.
When they arrive at the bugout, they see that all the food and water have been consumed and realize that Kelton’s older brother, Brady, must have moved into the bugout when he left his family’s house after high school. They still think they have one box of Henry’s water bottles in their truck, but when they open it they find only advertisement brochures. Henry tries to sneak away, but Kelton tackles him. They realize Henry has been lying to them all along so that they would keep him with them. They tie his hands and exclude him from decision-making
Kelton thinks they have one last hope of survival if they go to the San Gabriel Reservoir, but they are dangerously dehydrated. If they don’t get water within a few hours, they’ll collapse and die. When they stop the truck to investigate some loud music, Kelton discovers that two men with large weapons are drinking water openly beside their camper. He also realizes that the music and the water must be a trap. Henry steals the keys to the truck and runs toward the men. Kelton follows him.
Henry and Kelton have been gone for a few minutes when the two scary men approach the truck and announce to Alyssa that Henry sold the truck to them for a drink of water, and then ran away. When one of the men grabs Alyssa, Garrett bites him with enough force to leave a bloody wound. The other man aims a gun at Garrett, but Kelton arrives and shoots both men in the head, killing them.
Garrett faints from exhaustion, and Alyssa knows he is near death. Alyssa, Kelton and Jacqui go back to the men’s camp to retrieve their water but find that their campfire has expanded, burning their cooler full of water. Alyssa goes inside the camper and sees an old woman. Alyssa takes the woman’s water cup and runs to give it to Garrett, who revives before they drive away.
Kelton accidentally damages the gas tank of the truck with his poor driving, so the four kids must walk 1 mile to reach the reservoir. A fire appears in front of them, having spread from the men’s campfire. Instead of running around the fire, Jacqui runs right through it, trying to get to the reservoir.
Alyssa, Kelton and Garrett begin to walk uphill, around the fire, which only keeps growing and getting hotter. Kelton nearly collapses, but Alyssa refuses to let him rest. When they reach the top of the ridge, Alyssa sees that there’s no downhill slope, just a cliff face. They are trapped with a wildfire coming toward them. Alyssa realizes that her little brother may burn alive if she doesn’t shoot him first.
She holds the gun to Garrett’s head, and he asks her to kill him. Then suddenly, she hears a helicopter overhead. It’s a firefighting helicopter, which dumps a load of water on all three kids. Alyssa and Garrett drink up water that has fallen on nearby rocks, but Kelton has passed out and can’t drink. Alyssa fills up her mouth with water and brings it to him, little by little. The kids see the helicopter heading to the reservoir for a fresh load of fire-fighting water, which gives them hope.
Two weeks after the Tap-Out is declared over, life has returned to a semblance of normalcy for Alyssa and Garrett. They were airlifted away from the wildfire, received treatment for smoke inhalation and were reunited with their parents. Back in their neighborhood, they return to their school to finish the academic year. Uncle Basil survives the crisis and returns to live with them.
They see Henry on TV, swindling people again by pretending to be a hero who rescued trapped residents of a nursing home. Alyssa’s father prepares to drive her and Kelton to a hospital where a burn unit victim — Jacqui — has requested to see them. Civilization is slowly repairing itself.