The Devil's Mouth

Five friends, Sara, Max, James, Adrianne and Greg, travel to Thailand on a graduation trip funded by Sara. Sara's father died during a visit to Thailand, and she hopes to honor his memory by visiting the country and seeing the Devil's Mouth, a cave he loved.

On the cruise, Sara and Max discuss their plans after graduation. They had planned to move to New York, where Sara had secured her dream job, but Max reveals that she has accepted a job in Los Angeles and pressures Sara to move there with her instead. Later, Sara goes swimming while the others continue partying. Max forgets about her and allows the boat to leave, leaving Sara stranded in the water, where she is stung by several jellyfish. Sara is angry, but Max insists it was not entirely her fault. They eventually apologize, though their friendship is shown to be dominated by Max, who frequently controls their decisions while Sara struggles to stand up to her. That night, the group finds a half-eaten turtle washed ashore.

The next day, the group sets out for the Devil's Mouth with their cave guide, Wat. Sara has planned their route and suggests a safer, more frequently explored path, but Max insists on taking the more dangerous red route. The others eventually agree, despite Sara and Wat's disagreement. During the expedition, Sara learns that Max paid Wat extra to look after her, making her feel infantilized. Wat tells the group that a recent storm surge has flooded the caves with seawater, bringing marine life into the freshwater caves, where the animals later died. Wat repeatedly warns them to behave and to leave after reaching the Devil's Mouth, but Max insists on swimming deeper and staying longer. Adrianne injures her leg during the swim, and Sara treats her with first aid. Despite Wat's warnings to return so that they can provide Adrianne the help she needs, Max dismisses the injury and insists they continue.

While exploring the inner caves, Sara discovers the body of the diver they had encountered earlier. The diver's equipment later surfaces with shark teeth embedded in it, confirming that a shark is inside the cave. Wat urges the group to return, but Max refuses to do so without a plan. Wat tells her that his knowledge of the caves gives them their best chance of getting out safely, but Max continues to argue with him. Sara asks the group to follow Wat, but Max refuses. Realizing that staying with them puts his own life at risk, Wat leaves. The blood from Adrianne's injury attracts the shark.

The group attempts to make their way back through the caves, but a shark kills Greg, and Adrianne is severely injured, trying to save him. James identifies the shark as a bull shark, which can survive in both saltwater and freshwater, explaining why it survived the flooding. The survivors retreat onto a cliff face, where Sara notices that the map shows several narrow tunnels that the shark may not be able to enter. She suggests using them to escape, but Max insists they wait for Wat to return with help. Sara points out that they do not know whether Wat will survive, but Max refuses to reconsider. They soon hear Wat's screams and understand that the shark has killed him. Realizing that even an experienced Wat could not find his way out, Sara suggests taking the shortest and quickest route through a tunnel leading to the open ocean. She believes the brighter open water will make it easier to escape the sharks and survive. Max disagrees and insists they follow the original route, going back through the caves. She also suggests leaving Adrianne behind, so that they can make their way out and then send help to save her, but Sara and James are reluctant to leave her alone. The shark attacks again, causing Adrianne to fall into the water, and kills her.

Max insists that they take the route through the caves, against Sara's advice. Frustrated by Max's constant control, James tells her that she always makes decisions for the group and manipulates the others into agreeing with her. Max becomes angry and leaves on her own. Sara and James follow her, deciding to take the route Max suggested. James explains that they cannot let Max go alone, especially since Sara knows what it feels like to be left behind. Sara points out that she was abandoned, while Max is choosing to leave. James then tells her that she is partly responsible because she always lets Max take control.

After a long swim, they find a narrow, dry tunnel that requires them to crawl through it. While crawling, the tunnel suddenly collapses, causing Max to fall into the water below. Sara and James jump in to rescue her. During the swim, James becomes trapped between rocks and, unable to free himself, tells Sara and Max to leave him. They reluctantly continue, and James eventually suffocates. Sara encourages Max to keep moving as the shark catches up with them. She uses a floating body as bait to distract it, but realizes they have been swimming in circles and have returned to the area where Adrianne was killed. Sara again suggests taking the tunnel to the open ocean, but Max insists on continuing through the main route. The shark spots Max's headlamp, and despite Sara's warnings to hide among the stalagmites, Max swims ahead. When she climbs onto the cave wall after spotting the shark, it rams into the wall, causing part of the cave to collapse. Max injures her hand. Sara distracts the shark with a flare and throws it into its mouth, injuring it. Max loses consciousness, and Sara carries her through the cave. She eventually finds a small opening to the ocean. The two rest on a rock, where Max apologizes for controlling their friendship and never listening to Sara. She agrees to move to New York with Sara.

Sara reaches the opening and discovers that the climb to the surface and descent to the ocean are steep and covered with loose rocks. She manages to climb partway up and decides to make the descent at night when the tide is high. Since Max cannot climb with her injured hand, they agree that Sara will go alone and get help. At the last moment, however, Max insists on going with her.

As they climb, the shark attacks Max, causing Sara to fall back into the cave as well, as they are connected by a rope. Sara tries to rescue her, but the shark kills Max. Sara cuts the rope, separating herself from Max. Alone in the cave, she remembers Wat's warnings about the unstable cave walls. She lures the shark toward her, and the shark crashes into a wall, which causes part of the cave to collapse, crushing it. Sara then continues searching for a way out and discovers another opening leading toward land. Seeing signs of civilization outside, she climbs through the opening and cries out in joy.