The film opens with images of the Titanic’s departure from Southampton in April, 1912. In the present day, treasure hunter Brock Lovett leads a team of submersibles down into the Titanic’s wreck. He finds a safe containing a drawing of a nude woman wearing a necklace he is seeking, called “the Heart of the Ocean.” Brock receives a phone call from a 101-year old woman claiming to be the subject of the drawing, and he flies her out to his research vessel to hear her story.
Named Rose Dewitt Bukater, she explains to Brock and his team that she had boarded the Titanic in Southampton with her fiancé, Cal Hockley, and her mother Ruth. Thus begins the flashback which will be most of the film's narrative. We see Jack Dawson, the penniless artist with whom she will soon fall in love, winning tickets for the Titanic's voyage in a lucky round of poker in a nearby pub, and he boards the ship at the last minute. Rose describes the Titanic as “slave ship,” given how suffocated and unhappy she feels as Cal’s wife-to-be. After the ship departs from the harbor, Jack and his friend Fabrizio ecstatically rejoice at the ship’s bow. Rose dines in first class with other members of the upper crust, including Molly Brown, the shipbuilder Thomas Andrews, and White Star Line executive J. Bruce Ismay. Rose especially resents her mother and Cal’s controlling natures, and Ismay’s arrogance when describing the Titanic.
That night, Rose is about to commit suicide by hurling herself from the ship’s stern, when Jack happens upon her and convinces her to step back over the railing by saying he will jump in after her. White Star Line officials initially think Jack has attacked her, but Rose improvises a lie to exonerate Jack and conceal the motives behind her own behavior. Rose convinces Cal to invite Jack to dinner the following night. The next day, Rose strolls the deck with Jack, thanking him for his discretion. Initially shocked by his bluntness, Rose warms to Jack, especially impressed by his drawings. Molly lends Jack a tuxedo to wear to dinner in first class, where Jack charms the well-to-do with his carpe diem philosophies—all except for Rose's mother Ruth. After dinner, Jack secretly invites Rose to a raucous party below deck, where she drinks, dances, and feels liberated in the company of regular people.
The following morning at breakfast, after being informed of Rose’s behavior by his valet Lovejoy, Cal furiously scolds Rose. Ruth forbids Rose from seeing Jack again, reminding her that her marriage to Cal is crucial for remedying their family's precarious financial state. Jack tries to visit Rose in church, but is restrained by Lovejoy. Later that day, Rose strolls the decks with Thomas Andrews, noting that the ship only has lifeboats for half its passengers. Jack pulls Rose into a gym room and delivers an impassioned speech, worried that marrying Cal will extinguish the “fire” within her, but Rose tells him not to contact her anymore.
Later at sunset, Jack is standing at the bow of the ship when Rose approaches, saying she has changed her mind. Jack lifts her onto the railing, instructing her to close her eyes and spread her arms, and the two kiss. Rose invites Jack back to her first class cabin while Cal is at the smoking lounge, and asks him to draw her wearing only the Heart of the Ocean, which she retrieves from Cal’s safe. Jack draws her, and the two are later interrupted by Lovejoy. Jack and Rose sneak out the back entrance, and Lovejoy pursues them below deck. They run through the boiler room and wind up in a cargo area holding automobiles. They make love in one of the cars, and reemerge laughing on the ship’s deck, just as the ship is about to make contact with an iceberg.
The ship's lookouts ring the captain, and all over the Titanic, crew members work to throw the ship’s engines into reverse, to no avail. The ship collides with the iceberg, and Rose brings Jack with her to notify her mother and Cal about the collision, but Lovejoy and Cal frame Jack for stealing the Heart of the Ocean, and order the master-at-arms to arrest him. Below deck, alarmed third-class passengers see their cabins begin to flood, as above them, first class passengers remain largely oblivious to the severity of the accident. Thomas Andrews explains to Captain Smith, J. Bruce Ismay, and chief officer William Murdoch that the ship will sink in a matter of hours.
Rose shocks Cal and her mother by refusing to board a lifeboat, and instead goes searching for Jack, who has been handcuffed below deck under Lovejoy's charge. Thomas Andrews gives her directions through the crewman's passage to the rapidly flooding D-deck, where Rose finds Jack chained to a pipe. After failing to find a key, Rose runs through C-deck and finds an axe. She miraculously chops through Jack's handcuffs, and the two escape D-deck together. In C-deck, Jack helps the third-class passengers uproot a bench and ram through a gate preventing them from ascending to the upper levels.
Cal retrieves the Heart of the Ocean from his safe and stashes it in his coat. He finds Rose and Jack, and unwittingly gives Rose his coat with the diamond. He and Jack jointly convince Rose to board a lifeboat. Rose watches Jack as she descends, then leaps back aboard the sinking ship. Rose reunites with Jack, telling him, "You jump, I jump, right?" Enraged and jealous, Cal steals Lovejoy's gun and fires at Rose and Jack, sending them fleeing back down into the lower decks. He then realizes that Rose now has the Heart of the Ocean. Below deck again, Jack and Rose find a small child and try to rescue him, before being swept up in a current flooding the ship. They barely manage to escape the depths of the ship after Jack retrieves a pair of keys dropped by a fleeing White Star Line attendant.
Jack and Rose pass Thomas Andrews in the dining area, and he apologizes to Rose for not building a better ship. On deck, the ship's band plays while anarchy breaks loose. Cal finds a small, lost child and cynically uses her to board a lifeboat. William Murdoch, overwhelmed by managing the lifeboat triage, accidentally kills a passenger and then commits suicide. Captain Smith steps into the wheelhouse as it floods, killing him instantly. As the ship sinks by the bow, Jack and Rose run to the stern. The ship eventually snaps in half, and the front half sinks. Jack and Rose cling to the railing of the stern as the back half of the ship rises vertically into the air. Jack tells Rose to hold her breath as they finally go under.
Jack guides Rose to a piece of debris that she can use to stay afloat. Molly tries convincing the other people in her lifeboat to turn around and look for survivors, but is overruled. Jack makes Rose promise she will survive, and dies before the first lifeboat returns. Rose blows on a whistle to call the lifeboat, and is taken with the other survivors aboard the Carpathia the following morning. She registers the next day as "Rose Dawson" upon arriving in the United States. In the present day, Rose explains to Brock and the others that Jack saved her every way a person can be saved, and that Cal killed himself after the stock market crash in 1929. That night, Rose drops the Heart of the Ocean back into the sea. She goes to sleep and dreams she is back on the Titanic, kissing Jack, surrounded by smiling faces.
