Chris Lemons, Duncan Allcock, and David Yuasa are saturation divers maintaining gas lines in the North Sea. During a deployment, Lemons and Yuasa descend from a diving bell to work on a gas line manifold, while Allcock stays in the bell to monitor their status. A malfunction in the ship's Dynamic Positioning System causes it to drift, dragging the bell with Allcock inside. Lemons and Yuasa climb the manifold to avoid being dragged, but Lemons’s umbilical cord runs out of slack and becomes entangled, cutting off his air supply. Yuasa switches to emergency air and promises to return for Lemons, but the umbilical snaps, leaving Lemons stranded without air or communication.

On the ship, the crew struggles to fix the DPS malfunction. Meanwhile, Yuasa returns to the bell and regroups with Allcock. The crew on the ship locates Lemons using a remotely operated vehicle (ROV) but cannot lift him due to the tide. They eventually restore the DPS, and Yuasa rescues Lemons after 29 minutes without air. Lemons is unconscious and shows signs of oxygen deprivation but is revived with mouth-to-mouth resuscitation.

Lemons regains consciousness and appears unharmed despite the severe oxygen deprivation. He returns to port, reunites with his fiancée, and is later able to resume work after recuperating. In the epilogue, it’s revealed that Lemons suffered no lasting damage and returned to complete the job three weeks later.