At the local public pool, manager McGradey is putting up signs to let the public know that the pool will be closed for the holiday weekend. Among all the swimmers, Bree is enjoying herself, but she takes a break when it is time for her to check her blood glucose levels. Before jumping back into the water, she wonders if she should take off her diamond ring.
Meanwhile in the lost and found area, the janitor Clara is trying to steal money from the forgotten wallets but McGradey catches her red-handed. Now he regrets having spoken well of her to her parole officer and decides to fire her, but she will still have to finish today’s shift. In the parking lot, Bree’s sister Jonna is sitting in her car, wondering if she should go inside or not.
After lots of thinking, she decides to put her phone in the glove box and join her sister. Jonna used to be in rehab and the sisters haven’t hung out much lately, so Bree is happy they get to swim together again. At first Jonna is in a bad mood because of her bad home situation and the kids running around the pool bothering adults, but Bree convinces her to let go and have fun.
The rest of the afternoon goes well for the sisters, who get to enjoy their time in the pool until McGradey tells them it is time to leave. When Jonna points out the pool is supposed to be open until seven, McGradey explains they are closing earlier for the holiday weekend and reminds her to learn how to read. The sisters get out of the pool and while they dry themselves up, Bree remembers she wants to show Jonna her ring, but she can't find it in her bag.
Everyone else has already left and McGradey tells the sisters to hurry up, but they ignore him. Jonna checks the pool and notices that the ring is stuck in the metal grill, so Bree dives in to get it. She pulls as hard as she can yet she can’t take it out, so Jonna also jumps in to help her.
At that moment McGradey takes one last look at the pool area and sees nobody around, so he starts closing up. The girls are still struggling with the ring and Jonna's hair gets stuck in the process, causing the sisters to concentrate on untangling it and not notice the fiberglass lead slowly covering the pool. By the time they swim up it is too late, so now they are trapped.
Panicking, the sisters yell for help as they hit the pool cover, but unfortunately McGradey has already left. After arguing over what to do, Jonna starts feeling a bit nauseous and freaks out when she reaches the conclusion that McGradey left them here on purpose. Bree calms her down and they swim to the middle of the pool to try to push up the cover, but it is made of solid fiberglass and won’t even budge.
Bree’s next idea is to check all the seals and edges to find any weak spots, but there is nothing except for a pair of swimming trunks. Eventually they realize the cover has a condensation hole, but it won’t budge either, so Bree goes to remove the plastic cover from a hole in the pool wall. By breaking it in half, she creates a tool that can fit through the hole to try to saw it.
While Bree works, Jonna continues to complain and an argument ensues. Jonna can’t stand that Bree moved one while she was in rehab, and now Bree thinks Jonna threw the ring into the pool out of jealousy. A furious Bree dives in and finally gets the ring out of the grill while Jonna takes over the sawing only to accidentally cut her hand.
Bree comes back and points out that the grill could be heavy enough to break the fiberglass, but she isn't strong enough to take it off. Jonna refuses to help, but before another argument can begin, they hear a noise. It turns out Bree’s phone is ringing with a call from her fiance David, who leaves a message saying he will be attending a company dinner so he will get home late.
This means the sisters can’t depend on him to notice they are gone anytime soon. Desperate, Bree dives in again and tries to take off the grill, but it doesn’t work. Hours pass and as night falls outside, the lights go out in the building.
Bree can hear her phone ringing with David’s calls and Jonna keeps on complaining, this time about how she doesn’t like David. Suddenly Jonna notices Bree doesn’t look well and asks what’s the problem, causing Bree to confess she was diagnosed with diabetes while Jonna was in rehab. She needs to take her shot soon or she may fall into a diabetic coma.
Meanwhile Clara returns to the building, but she isn’t working very hard on finishing her chores. Back to the sisters, they are now taking turns sawing the hole. Jonna looks at the burnt marks on Bree's arm and wants to know what really happened the day their father died because she doesn't remember anything.
Bree tries to dodge the question, but ends up calling their dad a monster and explains what happened. That night their father came home drunk and lit a cigarette before passing out. The cigarette fell on his pillow and started the fire, but he never screamed because he was so drunk he couldn’t tell he was burning.
Bree had tried waking him up, only to get her arm burnt. At that moment Clara enters the pool room and hears Bree’s phone ringing. When she comes closer to steal it, she hears the girls yelling for help and asks her a few questions to buy time to empty Bree’s wallet.
However she stops herself when she notices the security cameras are on. Clara goes to the control room to turn the cameras off and bring the lights back, then she returns to the pool and asks Bree for her phone password, claiming she just wants to help. Bree gives her the number but instead of calling the police, Clara begins playing David’s messages.
Jonna begins yelling insults at her, so Clara drops the teasing and explains her situation. After being in prison, she has trouble finding work and she really needs money, so she will save the sisters if Bree gives her the PIN for her ATM. The sisters refuse to play her game, so Clara turns off the lights and the heater, threatening to let them freeze through the night.
Feeling desperate, Bree tells her the PIN and Clara leaves with her wallet. She doesn’t turn the heater back on though, so the sisters try to huddle for warmth. Hours pass and Bree’s body is aching badly because of the cold.
Jonna decides she has to try to take off the grill and dives in, but she is too scared and doesn’t even get close to it. When she resurfaces, she discovers Bree has fallen asleep, so she rushes to wake her up. Then Jonna admits she couldn’t even try and blames herself for the situation as she calls herself useless, but Bree reassures her she’s fine.
Afterward, Jonna goes back to sawing the hole with the shard, claiming it is the only thing that keeps her warm. Bree insists they should talk about their issues, so Jonna confesses she used to see monsters anywhere and that is why she tried to self-delete. To this day she still feels like she has a monster inside eating her, which represents the abuse her dad put her through.
Jonna says their father0's voice never left her mind. Suddenly the sisters are startled by something appearing on the hole, but it is just Clara. She is angry because Bree only has eighty bucks in her bank account and starts ranting, saying the girls aren’t special for having problems and that the world doesn’t owe them any favors.
Then she admits she may leave them there to die and goes to the control room. Opting for desperate measures, Jonna sharpens the plastic shard against the pool wall before pretending she is having a breakdown. She cries as she apologizes to Clara and claims to have something important to share, talking very lowly to make Clara come closer.
As soon as she rests her ear on the hole, Jonna stabs her with the shard. Clara screams in pain and rushes to the bathroom to check on her wound. Furious, she decides to get revenge by going back to the control room and turning on the automatic pool cleaning system, which causes the girls to start suffocating in chlorine.
While trying to stay afloat, the sisters try blocking the jets with their hands and the swimming trunks they found earlier, but the jets are too strong and push them back. Suddenly Bree loses consciousness and starts sinking, so Jonna has to push her up and hold her until she wakes up. It’s getting hard to breathe and the girls tell Clara that she is killing them, which finally makes Clara realize this is more than a mere prank and turns the cleaning system off.
Afterward Clar asks for Bree’s ring in exchange for their freedom. Ignoring Jonna’s protests, Bree gives Clara the ring, but Clara doesn’t let them out yet. She says she will do so when she is ready and turns the lights off again.
Moments later, the pool cover is finally moving and Bree can hear sirens outside. David bursts inside to save her but he can’t see Jonna, so he thinks she must drowning. Bree cries because she is too weak to save her, only to suddenly wake up and realize it was all a dream.
Then she checks on her sister and discovers Jonna is considering using the plastic shard to self-delete. Fortunately Bree manages to talk her out of it, promising that she will always be there for her. The girls apologize to each other and share a hug.
Meanwhile Clara is trying on the ring while Bree’s phone continues to blow up with David’s messages. His last one says he will call 911, so Clara returns to the pool to end this deal already. The sisters pretend that Clara is right and this happened for a reason, apologizing for calling her a monster.
Clara cries and goes to the control room to punch in the code, however it doesn’t work. After freaking out, she tells the sisters the bad news but refuses to call the police because she is afraid of going back to prison, explaining that it almost killed her the first time. Giving up, Clara leaves Bree's things on the benches and leaves.
Hours continue to pass and Jonna keeps on swimming to keep her body active and warm. However Bree is growing very weak and when Jonna comes to check on her, Bree feels so dizzy that she makes a confession. She always knew what their father did to Jonna and was disgusted by it, so she wanted to do something.
The night of the fire their dad had actually been awake and tried to move, but Bree held him down so he would burn to death and the girls would finally be free from his monstrous ways. Touched by the gesture, Jonna decides she must try harder for her sister, so she leaves her resting on the buoys and dives in to work on the grill again. She has to resurface to breathe, but after a few failed tries, Jonna gets an idea.
She takes the swimming trucks from earlier and wraps them around the grill's bars, which allows her to pull properly and without hurting her hands. The grill slowly starts to budge and after lots of pulling, Jonna finally takes it off and triumphantly takes it to the surface, only to discover Bree is unconscious. Jonna rushes to give her sister mouth-to-mouth and thankfully manages to wake her up.
Afterward Jonna uses the grill to hit the fiberglass cover on the hole area and it soon starts cracking until it finally breaks. The resulting space isn’t very big though, so Jonna gets hurt when she inserts her arm to break the pieces and create a bigger hole. Next Jonna has to drag Bree out because she is unconscious again, and once they are finally out of the pool, she rushes to Bree’s bag to get her medicine.
She injects it into Bree’s leg and after a few scary seconds of nothing happening, Bree wakes up for good. At that moment Clara returns and points a gun at them because she is afraid the sisters will tell the police about her. Jonna shares some heartful words about Bree being the only thing she has in this world, and this is enough to make Clara lower the gun and hand them back the ring.
Then Jonna uses Bree's phone to call the police and tells Clara to leave before they arrive. Moments later, Bree wakes up on a stretcher. After the paramedics check on her, Jonna shows her that she’s recovered the ring, saying they’ve finally killed the monster.
