Following the opening sequence, Airport '77 moves on to introduce the main characters, who are all VIPs on board the ill-fated flight. Among them are Philip Stevens (James Stewart), a wealthy art collector who is transporting his valuable collection to a new museum location, and Karen Wallace (Molly Picon), a spunky old lady who is traveling with her beloved dog. As they fly over the Bermuda Triangle, a group of hijackers, led by bank robber Eddie Bendix (Jack Lemmon), take over the plane with the intention of stealing the precious cargo on board.
After knocking out the passengers and crew with sleeping gas, the hijackers try to change course and land the plane on a remote island. However, during the descent, they crash into the sea and the plane sinks to the ocean floor. Fortunately, a small group of passengers survive, huddling in an air pocket at the rear of the plane. Among them are Karen and her dog, musician Steve (Robert Foxworth) and his girlfriend Eve (Kathleen Quinlan), flight attendant Julie (Olivia de Havilland), and a young boy named Robin (Gil Gerard).
While the passengers wait for rescue, they must contend with the rising water level, dwindling oxygen, and the deteriorating mental state of one of the hijackers, who becomes increasingly violent and dangerous. Meanwhile, on the surface, Philip Stevens and his team use cutting-edge technology to locate the wreckage and mount a daring rescue mission. They enlist the help of underwater salvage expert Frank Powers (Lee Grant) to guide a specially designed submersible to the plane and extract the survivors.
The climax of the film sees the two rescue efforts converging on the sunken plane, with the passengers fighting for survival against the elements and the hijackers. In a tense and emotional finale, the survivors are finally pulled to safety and reunited with their loved ones on shore.