After jumping off a yacht to swim, a group of friends suddenly realize that they forgot to put the ladder down and are trapped in open water. Will they be able to survive and climb back up? The story introduces us to a group of best friends - Amy, her boyfriend Dan, Zack, and Lauren.
They travel together, have fun, and film their adventures on camera. Noticeably tipsy, Dan picks up his camera and starts spinning around with it, which causes him to lose his balance and fall off a cliff. He gets out with minor bruises and returns to his terrified friends.
Five years pass. Amy, her husband James, and their little daughter Sara go to Mexico City to celebrate Zack's thirtieth birthday on Dan's new yacht. On the way, they are overtaken by Zach and Lauren on a motorcycle.
The whole company arrives at a yacht club where the yacht is parked. Amy is the only one of her friends who wears a life jacket and takes her time getting near the yacht. After Amy's father drowned while sailing together, she develops hydrophobia.
With her husband's support, Amy overcomes herself and climbs up on deck. The friends look around the yacht and find Dan in the cabin in the arms of his next girlfriend, Michelle. The group hoists the sails and sets off.
Zach and Lauren ask Dan how he could earn money for such a boat, but he guffaws and evades answering. Amy and James put their daughter to bed and leave a walkie-talkie in her cabin to hear when she wakes up. The party is in full swing: friends are drinking, dancing, and having fun.
Michelle offers the sun-warmed company a swim. They happily accept and dive into the water. Amy stays on the boat with Dan, and they go to the cabin to check on baby Sara.
Dan confesses to his ex-lover that he wants kids, too, and regrets being such a jerk in the past. Amy is happy with her husband, so she shortens the awkward conversation and suggests Dan return to the deck. Overboard, the friends are enjoying themselves.
Zach retrieves a seashell from the bottom of the sea and gives it to Lauren. Michelle flirts with James. Amy walks the deck with Dan and confesses that she hates her phobia and is afraid it will be passed on to Sarah.
In a pampering fit, Dan grabs Amy in his arms and, without listening to her hysterical cries, jumps into the water with her. She begins to panic, and flashbacks from the day her father died flash before her eyes. The friends try to bring her to her senses, but their voices sound to her as if from afar.
Suddenly, everyone realizes Dan didn't lower the ladder before jumping into the water. Only little Sarah is left on the boat, and all around them is the endless sea. James hums a lullaby to Amy to soothe his wife.
The rest of the group tries to climb back onto the boat by kicking off the water. They fail because her sides are too high. Michelle begins to panic.
Tired and cold, she desperately clings to the inflatable dolphin. Dan asks her to borrow the dolphin so he can use it to climb onto the boat. These attempts also prove unsuccessful, as the toy dolphin is too slippery and quickly deflates under Dan's weight.
The guys notice a flag waving on the stern, and Dan tries to grab hold of it. He manages to do it on his third try, but the flag is unstable and won't hold him. Amy comes to her senses, and everyone takes a breath to think about what to do next.
Michelle is hysterical again - she thinks a shark is swimming underneath them. Dan dives underwater but finds no one there. Despite this, they realize it's only a matter of time before the predators appear.
Everyone starts to pounce on Dan and blame him for what is happening. He realizes he screwed up but can't find a way out of the situation. Amy tearfully says they must find a way out because Sarah is left upstairs alone.
Michelle continues to lose her mind. Due to fatigue, she begins to drown and tries to take the lifejacket away from Amy. They push her away from Amy, and she starts having a nervous breakdown.
Lauren puts her on her back to calm Michelle down and reads a prayer aloud. The group spends some more time in the blazing sun. They run out of energy and are thirsty.
Suddenly, somewhere near the boat, they hear a cell phone ringing. The sound comes from Zach's dangling jacket. Dan manages to throw the jacket down, and Zach answers the call, but because the phone is soaking wet, they don't have time to pass anything on.
In complete distraught, Michelle snatches the phone out of Zach's hands, and it falls into the water, depriving them of an opportunity to try to reconnect. Dan suggests a new rescue idea - take off all their belongings and tie a rope from them to throw it on board. At this point, the guys notice a boat rushing past them at high speed.
The friends desperately try to signal the passengers, but they don't understand the guys' plight and wave their hands in response as they pass by. They have to go back to Dan's plan. Using the knife they found in Zack's jacket, they cut the clothes into pieces and knit a rope from them.
James asks Dan if it is possible to get to the boat from below, but he doesn't know. So he checks the boat himself and, taking the knife from his friends, dives under the water. Meanwhile, the group manages to throw a homemade rope on board, and now Zack tries to use it to climb up.
This attempt also proves futile - the rope breaks, and Zack falls back into the water. James is also haunted by failure. During another dive, he accidentally loses the knife.
To save the sinking knife, he swims too deep and hits the bottom of the boat during his ascent. James swims out, and Lauren examines him. Blood from his ear reveals that he has suffered a head injury and must get to a doctor immediately.
This becomes the last straw for Zach, and he decides to resort to more drastic rescue methods. He takes a knife from James and starts punching through the side of the boat to use it to get upstairs. Dan rushes to his friend in hysterics and tries to stop him, as he doesn't want him to damage the boat.
During the altercation, Dan accidentally stabs Zack in the stomach. The situation gets entirely out of hand. From the radio comes the hysterical crying of Sarah, who is already awake and not seeing her parents around.
Because of the severe wound and lack of medical attention, Zach slowly and painfully dies. Michelle drifts away from the group and, finally losing her strength due to fatigue, goes down. Dan tries to save her but realizes it is too late.
Grunting and panting, Zach tries to make a joke. He remembers their trip five years ago and wishes Dan had brought a camera this time. His last memory is of Lauren - he remembers how beautiful she was the last time they saw each other.
After these words, Zach becomes silent, and his body goes limp right into Lauren's arms. She begins to sob hysterically. This time, James's nerves are fraying.
He lashes out at Dan and accuses him of valuing the boat more than the lives of his friends. Amy tries to calm her husband down, but he is even angrier that his wife is defending her former lover. James's abrupt movements make him begin to vomit.
Dan admits to his friends that he lied to them about the yacht and his financial situation. It turns out that Dan arbitrarily borrowed the yacht from his boss, from whom he works as an errand boy. He posed as the owner of the boat to lure beautiful girls like Michelle.
He wanted to brag to his old friends about his luxurious life so they wouldn't guess he had never accomplished anything over the years. That's why he didn't want the knife nicks left on his yacht; now, his best friend is dead. From the realization of what he had done, Dan begins to cry.
Lauren confesses that she was in love with Zach but never got around to telling him. Amy convinces her friend to let his dead body go. Several more hours pass in the open sea.
Exhausted, everyone keeps their balance on the water and recalls moments from their lives. James' condition grows worse, and he begins to rant. Lauren, unable to stand the inactivity, says goodbye to her friends and swims away, hoping to get to shore and call for help.
Little Sarah continues to cry. Amy tries to shout to her through the radio, but the baby does not calm down. She remembers Zach's idea of getting onboard with a knife and is about to dive into the water to find it.
Dan stops her and goes underwater himself to make it up to his comrades somehow. It's getting dark. Amy is left alone on the water with James.
He periodically passes out and tells his beloved that he is tired and has a headache. It starts to rain, and James dies quietly. Amy screams, distraught at the pain of losing her beloved.
Dan hears her cries, but he continues to dive into the water to find the knife to save them. Night falls. Finally accepting her husband's death, Amy releases his body.
As she says goodbye, she hums the lullaby they sang to her daughter together before bedtime. Dan emerges from the water, screaming in pain. His arm is cramped, and Amy shares her vest with him to help him swim to the boat.
Meanwhile, the storm intensifies, and the chances of survival in such conditions are incredibly slim. Dan, feeling helpless, slams his goggles into the stern of the boat with all his might. The goggles shatter, and all that's left is the glass, which hurts his hand.
That's when he gets an idea of how to use the glass to save himself. Dan pushes the glass into the gap between the board and the folding gangway and holds on to it with his hand. He calls out to Amy, who, using Dan's hand as a stepping stone, tries to climb aboard the boat.
She tries to grab hold of the board, but the heavy rain makes her hand slip, and she falls into the water. Dan asks her not to give up and try again. She gets on his shoulders and tries to climb up again.
Dan struggles to hold on to the sharp shard of glass, screaming in pain every time Amy steps on his arm. Amy's hand slips again, and she falls. Gathering all her remaining strength, she makes another attempt and finally manages to cling to the side and climb to the deck.
Overwhelmed with emotion, Dan laughs and shouts for joy. Amy lowers the gangway and immediately runs into the cabin to her sobbing daughter. Dan clings to the gangway but stops.
He is faced with a choice: to follow his friend and save himself or accept punishment for what happened to his friends through his fault. Taking Sara in her arms, Amy soothes the little girl and lies down next to her, glad that the nightmare they experienced is over. Moments later, she realizes that Dan never came aboard.
She walks out on the deck and sees a lifejacket bobbing lonely on the water. In the distance, Amy notices Dan's motionless silhouette. She unsuccessfully tries to shout out and get the life jacket to him.
She realizes that her guilt made Dan decide to drown himself. Amy recalls the same way her father drowned in front of her when she was a child. To prevent another tragedy, Amy overcomes her phobia and swims after Dan.
He goes down, but Amy tries to pick him up. Frames of what's happening are intertwined with what happened in her past. Back then, little Amy was also trying to save her father.
It is morning. A fishing boat approaches the yacht. The fisherman swims around the empty vessel and tries to shout to the passengers.
After hearing no answer, the fishing boat sails away. Sarah wakes up in the cabin and begins to cry. At the end of the story, we see Amy standing on deck, looking out into the distance.
Dan lies at her feet unconscious, whom she managed to save. The fishing boat is no longer in sight. Only the sounds of the engine can be heard in the distance.
But she has survived and can now return home.
