Millionaire philanthropist Philip Stevens (James Stewart) arrives at his palatial estate and holds a media conference to discuss a flight containing his hand-picked guest list of rich and famous friends, which will soon be arriving in Florida. His luxury aircraft, a Boeing 747 outfitted with high-end accommodations, is packed with his collection of priceless artwork to be installed at his home, which has been converted into a new museum where Stevens hopes to share the treasures with the world.The plane, piloted by Captain Don Gallagher (Jack Lemmon) lands in Washington. Gallagher meets the plane's lead production manager Stan Buchek (Darren McGavin) upon landing, and tells him the aircraft handled perfectly, and as they observe the museum pieces being loaded into the cargo holds, wonder whether the collection or the plane is more expensive. Meanwhile a mysterious pair of men-an airline employee and a civilian--stand at the airport gift shop and quietly switch suitcases as they meet at a magazine rack, never making eye contact or speaking. The employee then enters a locker room and assumes a disguise with a mustache and wig, finding a gun in the bathroom paper towel dispenser, and joins the ground crew as the cargo is loaded. Meeting up with a companion on board later, canisters of CR-7 gas are connected to the air supply, rigged to be activated manually.Gallagher finds Eve Clayton (Brenda Vaccaro) in an office. They have been a couple for some time, and he finds the courage to bring up marriage, but she says Stevens has asked her to work in Geneva for him, which would put distance into their relationship. He wants a wife and family, and she wants her career.That evening the guests begin to arrive and board the aircraft, including Benjy (Anthony Battaglia), whose mother is Stevens' daughter Lisa (Pamela Bellwood). She is taking her son to meet his grandfather for the first time, but her own relationship with her father is strained. Emily Livingston (Olivia de Havilland) boards with her personal assistant Dorothy (Maidie Norman). She joins several art critics she knows and, after the men and she exchange banter, she sits with them as their fourth and engages in a challenging game of poker. Jane Stern (Arlene Golonka) arrives with her daughter Bonnie Stern (Elizabeth Cheshire), who was chosen for the flight to present an artwork her schoolmates designed for Stevens' museum.A film is played of Stevens greeting the passengers and urging them to enjoy their flight. Bob Chambers (Robert Foxworth), the main conspirator, will be the co-pilot for the flight that evening, and he and Gallagher settle in the cockpit, while conspirators Wilson (Michael Pataki) and Banker (Monte Markham) are among the crew members. Gallagher appears on the screen to invite the passengers to view their taxi down the runway as the plane, now known as 2-3-Sierra, begins its flight.At the bar, blind pianist Steve (Tom Sullivan) plays and sings a song for corporate assistant Julie (Kathleen Quinlan). The children challenge Dorothy to a tabletop video game while a woman leaves to find two men in the nearby office. Frank Powers (Gil Gerard) discusses a future underwater farming excursion with diver Martin Wallace (Christopher Lee), whose demeanor grows sour when the woman, his wife Karen Wallace (Lee Grant), enters the room in an inebriated state and shows off the bottles of booze she has collected. When Martin leaves, Karen teases Frank with threats to tell her husband about their affair, one which he much regrets. Eve talks to Philip from the cockpit and informs him that Lisa and Benjy are aboard, and he decides to head out to meet them. Eve then tells Gallagher she has thought things over and wonders if they can discuss their relationship in Palm Beach later. He agrees.A drink arrives for Emily, along with a calling card bearing the name of her old flame Nicholas St. Downs III (Joseph Cotton), she sees him across the lounge area and is happy to reunite with him after decades apart. Eddie (Robert Hooks) tends bar and tells Eve his wife is expecting twins, and she encourages him to call home to check on her, then speaks with Lisa in private. Determined to become independent, Lisa wants to break the news to her father that she won't be dominated by his power and position any longer, but Eve informs her that Philip is terminally ill and has months to live, and urges her to restrain herself. Julie and Martin are at the bar and discuss the sea, which enthralls him as a diver, but she admits water frightens her. Karen interrupts their talk and tosses a drunken barb at Julie, who leaves, and she and a miserable Martin exchange insulting banter.The conspirators begin their coup. The security guard is knocked out, but is hit too hard and dies instantly, so they move the body out of sight. Gallagher is lured from the cockpit to tend to a sick passenger and is subdued by a blow to the head. They secure oxygen masks to their faces, the sleeping gas is released into the cabin, and the passengers lose consciousness. Chambers takes the controls and the plane descends below radar, heading into a fog bank, reducing visibility to less than a mile. He changes the flight path over the Bermuda Triangle, taking the craft off course at 600 mph. The air traffic controller radios the plane from Palm Beach when they lose the radar signal, but he ignores it. The plan is to remove the artwork and head to South America with their black market profits, while the passengers awaken to an empty plane on deserted St. George Island.Minutes later, as Chambers looks at the vista ahead, a large offshore drilling platform emerges from the haze, and they are heading straight for it. He pulls back on the yoke and tries to turn, but the wing clips the derrick and the engine catches fire. Chambers immediately hits the fire extinguishing button and the flames go out momentarily, but the sudden loss of airspeed threatens to stall the airplane. As the engine reignites, the stall alarm goes off and the tail of the plane hits the water. The passengers awaken and start to scream and panic. Chambers struggles to keep the plane airborne, but finally it hits the water violently and stalls in the ocean. Banker is in the hold securing the art for the transfer when a cargo container slams into and breaches the bulkhead, the ocean bursts in and sends the container back to pin him to a wall, and the surging water drowns him. Wilson is killed when he enters the cockpit and is slammed into the flight panel, and Chambers is knocked unconscious at the controls. As passengers recover from the initial shock of impact, Karen looks out a window and alerts everybody as the plane begins to slip beneath the waves, settling on the ocean floor some 100 feet below the surface.The ocean bottom is fortunately above the depth that would crush the fuselage, and Gallagher calms the passengers, informing them that they have been tracked and will be rescued soon. Buchek suffers an impact fracture of his arm while trying to protect Bonnie, who has been injured under the bar. Emily looks after Dorothy, who took a blow to her head. Gallagher and Buchek ask a passenger, Dr. Williams (M. Emmett Walsh) to help. The doctor spends time tending to the wounded, though he later confesses to the lead flight attendant that he is actually a veterinarian for Stevens and is used to tending horses. Gallagher and Buchek examine the plane and find some cargo areas flooded, but others remain airtight for the time being. How long the plane will hold up, and the lack of oxygen, are concerns.Their attention is drawn by Eddie, who has suffered a severely broken leg, and Nicholas helps them take him into the main cabin to rest. Martin also offers to help, stopping momentarily to calm Karen but stressing to her that she will be fine while he is away. Gallagher and Buchek interrogate Chambers in the office when he regains consciousness and hear the confession of the conspirators' plans. They put him on watch in the passenger area.Steve the pianist is seriously hurt, Julie stays with him and, when he touches her face as a way of imagining how she looks, she gives him her earring as a token, and he says he loves her. When he requests a drink of water, in the moments it takes to bring it to him, she finds he has succumbed to his injuries. Meanwhile, Lisa assures Benjy that Bonnie will be okay.Ships and planes are sent out to search for the downed flight under the guidance of Joe Patroni (George Kennedy), and he assures Stevens that the plane will be found. With the crash happening far from their expected flight path, Gallagher tells Eve they are on their own. Buchek examines growing damage to the fuselage and finds more airtight areas being breached by water, and the passenger compartment begins to flood.As Emily comforts Julie, she hears a droning sound outside and asks all the passengers to be quiet. Martin recognizes the sound as that of an approaching ship, which brings them hope, but the ship's crew do not know the plane is submerged near them and pass the crash site. Karen lashes out at Gallagher, blaming him for their situation. Gallagher and Buchek find the only inward opening door of use on the plane, located in a rear cargo hold which has remained dry, and they wire an electrical circuit inside. Gallagher announces to the passengers that he plans to exit the plane from that airtight compartment and send a life raft to the surface with a homing beacon to alert search parties to their location. Martin volunteers as an experienced scuba diver to join him and double their chances of success, and Gallagher agrees.Buchek shows Gallagher how to touch two wires together to create the spark needed to unlock the cargo door. Karen tries to stop Martin from taking the risk, saying they should think about themselves, but he argues that selfishness sent them into their predicament, and he kisses her before he joins Gallagher. Buchek wishes Gallagher luck, leaves the hold and the hatch is shut and locked. Placing their oxygen masks to their faces, Gallagher notes they can get two minutes of air to breathe as he joins the two wires once. The door creaks but does not open, so he tries again several times, finally removing his oxygen mask to investigate the connection. Martin steps over to the door, and before Gallagher can warn him, it bursts open, striking Martin and throwing him across the cargo hold in a wave of ocean water as the cabin floods. Gallagher swims to a glassy-eyed Martin and tries to revive him with oxygen, but he died on impact and the effort is futile, so he grabs the raft and exits the hold into the open ocean.Karen and the others watch in horror as Martin's body floats away. Several passengers try to settle a shocked Karen as Gallagher swims skyward, pulls the inflation device and sends the raft to the surface ahead of him. Swimming to it when he surfaces, he boards and activates the beacon, then sits helplessly hoping for a quick response. A Navy plane investigates the beeper which has suddenly appeared on radar, indicating the flight is 100 miles off course.On the airplane, Karen is obviously in an alcohol-fueled state of shock from losing her husband. She absently preens in a mirror, then approaches the main hatch and attempts to open it, which would flood the plane. Eve approaches and tries to calm her down, but they begin to struggle. Karen pounds at Eve, who finally has to punch Karen and knock her out to prevent a catastrophe.A jet pilot reports to base that he has spotted the plane, which is completely underwater at the coordinates given. Gallagher shouts with joy as the passing jet dips a wing in acknowledgment. The Navy discusses the situation, and it is decided to raise the plane as they would a submarine in a salvage operation. Patroni guides the Navy in how best to raise the aircraft without splitting it in two, while Stevens is sent to meet with the survivors.The USS Cayuga arrives at the site, and a crew comes by boat to retrieve Gallagher, who talks with Patroni as the rescue is coordinated, stressing that the passengers are running out of time. With additional salvage crew members and equipment, the vessel sends divers (Gallagher also volunteers to help with his amateur diving experience and knowledge of the aircraft) below to the plane. A diver writes to the passengers on a tablet and shows it to them at a window, indicating that they should secure themselves as the crew attempts to lift the plane from the water. Divers run heavy straps under predetermined points under the fuselage and attach them to huge bladders to fill with compressed air to raise the plane.The crew aboard the inflation boat begin sending air down to the bladders, which inflate and rise, lifting the plane from the ocean floor, but another airplane door opens under the change in pressure, water begins to fill the plane, and one bladder breaks away. The flood sends some passengers spinning in its wake. Karen is swept away toward the back of the plane and drowns. Buchek grabs Benjy and prevents him being swept away as well. Chambers is killed when debris from the flooding strikes him.As the boats and crew circle and watch at the surface, the plane breaks through the water, and rafts-one with Gallagher aboard--pull up to the hatch as flight attendants ferry Eddie, Bonnie and Jane and the other wounded passengers aboard. Emily and Nicky see that Dorothy did not survive the ascent and reluctantly leave her. The able-bodied survivors rush out to waiting boats, but when a second strap holding a bladder keeping the aircraft buoyant gives way, Eve is trapped with the water rising at her only exit. Gallagher wades into the plane to her side and they climb the stairs to the cockpit to exit by the door there, and jump into the ocean. They are lifted by helicopter to safety. The remaining bladders holding their former prison give way and Flight 2-3 Sierra disappears again underwater.On-board the Navy ship, Philips greets Lisa and Benjy, the injured are airlifted away, and Gallagher and Eve reaffirm that they have a date in Palm Beach that night as they land and greet the other grateful survivors on the Cayuga's deck.