Danielle tries to reach over her shoulder and turn her air back on, but his hand is cupped around the valve, and even with near-death desperation she isn't strong enough to pull it loose. Her lungs are aflame with the need to breathe, she can feel the strength going out of her limbs. What is left of her vision, at this dark depth, begins to blur.

Then she remembers her "ocky", her second regulator, the backup with the long hose, in case the primary regulator fails or she needs to let somebody else buddy-breathe. Barely conscious, she sweeps her arm up in a wide backwards circle, just as her dive instructor taught her so many years ago, until her arm reaches the ocky's hose. Her fingers find the mouthpiece, and she spits out the primary regulator and brings the ocky to her mouth. There is enough air contained in its hose for a single deep breath. The onset of oxygen is immediate, the world's fastest and most powerful drug, her vision clears and she can think again. For one more breath. Thirty seconds.

They are very deep, so deep Danielle can hardly see at all, but she can tell that some dark fluid clouds the water around her. Blood, Laurent's blood, she shot him with the speargun. That explains why he is content to hold her and wait for her to pass out or start breathing water. It would be nice if the blood were to draw sharks to eat him in the next twenty seconds, she thinks crazily, but she probably can't count on that. But wait. He too has an ocky. And he is wounded. Keeping one arm wedged between her neck and Laurent's headlock, she reaches up with her free hand, locates his ocky, pulls it to her mouth, and sucks air in greedily. He doesn't dare cut this air supply off, now they are both breathing from the same tank.

Laurent releases his armlock and tries to pull his secondary regulator from her mouth, but Danielle, expecting that, spins to face him, so that her face is against his belly, and wraps her arm around him tightly. Then she recoils as her hand hits something in the water near his upper thigh, something so sharp it pierces her glove and her skin before she pulls back. His whole body convulses and she hears his groan through the water. The spear. She shot him through the leg. Heedless of her own bleeding hand, Danielle reaches out, grabs the shaft of the spear, and rocks it back and forth. Laurent releases her immediately, tries to get away from her, but Danielle hangs onto him like a limpet. Her face is right up against his gauges. The dials are fluorescent but she still has to squint. They are 240 feet, 70 metres, beneath the surface. 130 feet is the accepted safety limit for tanks filled with normal air.

She has never been this deep before. Few non-professional divers have. She has to ascend. She releases Laurent and inflates her BCD. He is still sinking, so fast that he must have put on both Sophia's and the South African's weight belts, so he would fall on her faster when he jumped from the boat to ambush her. But he was not quite fast enough. And with a spear through his leg he will neither make it back onto the lifeboat nor survive long enough to be rescued. She has won -- but then there is a sudden lurch, something grabs her from behind, it feels a little like a shirt caught on a loose nail, and she is falling back down. She inflates her BCD to full but she is still sinking. She reaches down, unhooks her weight belt, and lets it fall to the ocean floor, but that only slows her rate of descend. 250 feet now. She understands. Laurent has made the same calculation she has, that he can't escape. He grabbed her BCD, determined to take her down with him.

Only one last chance. Danielle rips open the velcro strap around her waist, takes the deepest breath she can manage, reaches for the plastic clips on her BCD's shoulder straps, and pinches them open. The vest flies open, releasing her like an sprung trap. She opens her mouth and lets the regulator out, and floats freely up through the ocean, separated from all her scuba gear, as Laurent falls further into darkness.

She is buoyant, but not like a cork. She wants to swim for the surface, kicking as hard as she can, but she knows the important thing to do, to give herself the best chance of avoiding the bends – and the bends will likely kill her if they hit before help arrives – is to ascend as slowly as possible, without running out of air and drowning. Her ascent to the surface must take at least two minutes.

The air in her lungs right now has been compressed to one-eighth its normal volume by the pressure of seventy metres of ocean. This same air will expand like a balloon as she goes up. If it expands too slowly, she will use all the air and run out of breath; if too quickly, her lungs will rupture from the pressure; but if Danielle is careful, if she breathes out constantly but very slowly, if she ascends at exactly the same speed as the air bubbles rising around her, this single breath can last for more than a minute, replenishing itself by growing as the pressure decreases. She makes a sound in the back of her throat, to ensure that she keeps her airway open at all times. She wants to breathe, she has never before gone two minutes without breathing, but she realizes halfway up that she doesn't really need to, it is habit alone that makes her desperate to inhale. She doesn't time it quite right. She runs out of oxygen forty feet beneath the surface and spends the last fifteen seconds of her ascent gagging for breath, forcing herself with iron discipline not to kick for the surface.

When she emerges from water into air, it is a rebirth. She takes what feels like her very first breath. The salt air tastes like nectar of the gods. The lifeboat is nearby. Danielle treads water, shaking her head, coming back to where she is and what she's doing, to reality, to life. She realizes that her whole desperate underwater battle with Laurent, and her epic single-breath ascent, occupied fewer than five minutes of her life. A life which looks as if it will continue for some time. Keiran is already in the lifeboat. He helps her aboard. Sophia lies on the floor, pale and weak, coughing and twitching. Vijay's body is nowhere to be seen, Keiran must have pushed it overboard.

"Smoke inhalation," Keiran says. "He pushed her out. I saw it. She needs a doctor."

Danielle nods. "I might too." Typically the bends take up to 24 hours to manifest.

"Let's hope your SOS got to someone."

Keiran paddles. Danielle spends a few minutes in the prow, watching the waves around her with a careful eye, but their smooth patterns remain unbroken. No sign of Laurent. Of course he could stay down there for a good hour, maybe more, with two tanks at his disposal. But wounded as he is, with a spear through his leg, in the middle of the North Pacific – he won't make it. Except it is terribly easy to believe that he might.

Then Danielle sees something break the water, something that makes her smile with relief and triumphant revenge. Not Laurent. A dorsal fin. A shark. A big one if she is any judge.

"The strong inevitably eat the weak," she murmurs to herself. "Isn't that right?"

"What?" Keiran asks.

"Nothing," she says. "Paddle us home."

But they don't make it to the Oregon coast. It only takes an hour for the Coast Guard helicopters to show up. By that time, a steady cleansing rain has blown in, and agonizing pains have already begin to stab at Danielle's knee and shoulder joints.