Two children, Jacques Mayol (Bruce Guerre-Berthelot) and Enzo Molinari (Gregory Forstner [fr]), have grown up on the Greek island of Amorgos in the 1960s. Enzo challenges Jacques to collect a coin on the sea floor but Jacques refuses. Later, Jacques’ father — who harvests shellfish from the seabed using a pump-supplied air hose and helmet — goes diving. His breathing apparatus and rope gets caught and punctured by rocks on the reef and weighed down by water, he drowns. Jacques and Enzo can do nothing but watch in horror as he is killed.
By the 1980s, both are well known freedivers, swimmers who can remain underwater for great times and at great depths. Enzo (Jean Reno) is on Sicily now, where he rescues a trapped diver from a shipwreck. He is a world champion freediver with a brash and strong personality, and now wishes to find Jacques and persuade him to return to no limits freediving in order to prove he is still the better of the two, in a friendly sports rivalry. Jacques himself (Jean-Marc Barr) works extensively with scientific research as a human research subject, and with dolphins, and is temporarily participating in research into human physiology in the iced-over lakes of the Peruvian Andes, where his remarkable and dolphin-like bodily responses to cold water immersion are being recorded. Insurance broker Johana Baker (Rosanna Arquette) visits the station for work purposes and is introduced to Jacques. She secretly falls in love with him. When she hears that Jacques will be at the World Diving Championships in Taormina, Sicily, she fabricates an insurance problem that requires her presence there, in order to meet him again. She and Jacques fall in love. However none of them realize the extent of Jacques’ allurement with the depths. Jacques beats Enzo by 1 meter, and Enzo offers him a crystal dolphin as a gift, and a tape measure to show the small difference between Jacques’ and Enzo’s records. Johana goes back home to New York but is fired after her deception is discovered; she leaves New York and begins to live with Jacques. She hears the story that if one truly loves the deep sea, then a mermaid will appear at the depths of the sea, and will lead a diver to an enchanted place.
At the next World Diving Championships, Enzo beats Jacques’ record. The depths at which the divers are competing enter new territory and the dive doctor suggests they should cease competing, but the divers decide to continue. Jacques is asked to look at a local dolphinarium where a new dolphin has been placed, and where the dolphins are no longer performing; surmising that the new dolphin is homesick, the three of them break in at night to liberate the dolphin and transport her to the sea again. Back at the competition, other divers attempt to break Enzo’s new record but all fail. Jacques then attempts his next dive and reaches 400 ft (122 m) breaking Enzo’s world record. Angered by this, Enzo prepares to break Jacques’ new world record. The doctor supervising the dive warns that the competitors must not go deeper – based upon Jacques’ bodily reactions, at around 400 ft, conditions, and in particular the pressure, will become lethal and divers will be killed if they persist in attempting such depths. Enzo dismisses the advice and attempts the dive anyway, but is unable to make his way back to the surface. Jacques dives down to rescue him. Enzo, dying, tells Jacques that he was right and that it is better down there, and begs Jacques to help him back down to the depths, where he belongs. Jacques is grief-stricken and refuses, but after Enzo dies in his arms, finally honors his dying wish and takes Enzo’s body back down to 400ft, leaving him to drift to the ocean floor. Jacques – himself suffering from cardiac arrest after the dive – is rescued and brought back to the surface by supervising scuba divers and requires his heart to be restarted with a defibrillator before being placed in medical quarters to recover.
Jacques appears to be recovering from the diving accident, but later experiences a strange hallucinatory dream in which the ceiling collapses and the room fills with water, and he finds himself in the ocean depths surrounded by dolphins. Johana, who has just discovered she is pregnant, returns to check up on Jacques in the middle of the night, but finds him lying awake yet unresponsive in his bed with bloody ears and a bloody nose. Johana attempts to help him, but Jacques begins to get up and walk to the empty diving boat and gets suited up for one final dive. Desperately, Johana begs Jacques not to go, saying she is alive but whatever has happened at the depths is not, but he says he has to. She tells Jacques that she is pregnant, and sorrowfully begs him to stay, but finally understands he feels he must go. The two embrace and Johana breaks down crying. Jacques then places the release cord for the dive ballast in her hand, and – still sobbing – she pulls it, sending him down to the depths he loves. Jacques descends and floats for a brief moment staring into the darkness. A dolphin then appears and – dreamlike – Jacques lets go of his harness and swims away with it into the darkness.