The film begins by informing viewers about the 20,000 miles of pipeline that snake beneath the world's oceans, maintained by saturation divers who often work in conditions that go as deep as 1,000 feet. This job is considered one of the most perilous in existence. We are introduced to the harrowing scene of an unconscious man lying at the bottom of a work station, setting the stage for the intense narrative that follows.
In Aberdeenshire, Scotland, we meet Chris Lemons, portrayed by Finn Cole, who is preparing to embark on a diving rotation in the dangerous waters of the North Sea. His fiancée, Bobby Rainsbury as Morag, is acutely aware of the job's risks and expresses her concern for Chris. Despite her worries, he reassures her, claiming that his profession is “pretty cool.”
As Chris navigates to the harbor, he meets his diving team: longtime friend Duncan Allcock, played by Woody Harrelson, and the seasoned diver Dave Yuasa, portrayed by Simu Liu. Tensions arise when Chris discovers that Duncan is being ousted after two decades with the company, making this rotation his last.
The crew sets sail, only to confront the raging elements of a storm. While Chris and Dave go underwater to commence their task, Duncan remains in the diving bell monitoring their situation. As the tempest intensifies, the ship's Dynamic Positioning System (DPS) fails, resulting in the vessel drifting precariously. In a frantic bid for safety, Chris and Dave race back to the diving bell, but disaster strikes when Chris's air supply cord snaps, leaving him stranded.
Amidst the chaos, Captain Andre Jenson, skillfully played by Cliff Curtis, scrambles to devise a plan to rescue Chris while simultaneously preventing an environmental catastrophe that could arise from their drifting anchor. The crew works in unison, with efforts to restore power to the ship and regain control.
Underwater, Chris faces dire conditions as limited visibility and dwindling oxygen threaten his survival. In the fading moments of consciousness, he envisions Morag's face as a source of strength, especially as she sends him encouraging video messages. Just as hope seems lost, a crew member identifies Chris's location using a remotely operated vehicle (ROV), revealing that he is alive but suffering badly from oxygen deprivation.
With the DPS now rebooted, Dave ventures back outside the bell to reach Chris. After grappling with the overwhelming challenges, Dave succeeds in locating and bringing Chris to safety, though Chris has endured nearly half an hour without oxygen. In a moment of shared relief, Duncan is initially concerned that Chris may not make it, but his fears dissipate when Chris miraculously speaks.
Ensuring Chris receives immediate care, he is placed in a saturation chamber for a momentous three-day decompression period, while Duncan and Dave remain at his side, providing support throughout the ordeal. On the third day, Chris is finally released from the chamber and has a heartfelt reunion with Morag, who keenly realizes the gravity of the incident without needing details. Chris assures her that his resolve to return to her kept him alive.
The film concludes with a dramatic epilogue revealing that Chris was submerged without oxygen for a staggering 29 minutes yet emerged without lasting physical or mental damage—something experts continue to find unexplainable. Three weeks later, Chris rejoins Duncan and Dave for another rotation, and the audience is treated to real-life footage of Chris's wedding, in which Duncan is present. The final moments capture him confidently diving back into the depths, underscored by a fellow crew member's humorous reminder to “Don’t fuck it up this time.”
