The film starts in the year 1997. Two small submarines are diving down to the wreck of the Titanic. Treasure hunters get up a safe from an apartment because they search for a special necklace called "the Heart of the Ocean". It is a blue heart-shaped diamond which makes the necklace so special. The treasure hunters are very disappointed because they only find a drawing showing a nude girl with the necklace. The elderly lady Rose watches TV and sees a report about the hunt for the diamond. So she calls the hunters; they invite her to come to their ship because she tells them that she is the girl in the picture. They hope that she can give them some information. The hunters show her a simulation from the sinking of the ship, then she starts to tell her story about the sinking. Rose, the 17-year old girl boards the ship with her mother and her fiancé Cal, a rich member of the high society. She is sad and some kind of "bought"; her family lost all the money, so she must marry a rich man. Besides that it is the Edwardian Age, the time where men dominate the women and treat them as if they were only some dolls. She wants to commit suicide by jumping of the ship, but Jack Dawson rescues her. When others come by it looks like he tried to rape her, but then it gets all cleared up and he is invited to dinner. (The unsinkable) Molly helps Jack, she gives him a suit because Jack's clothes are not the best. At dinner, Jack is only accepted by the people who do not know about his origin. Rose's mother sees that there is something between Rose and Jack. Rose spends the evening with Jack and has a lot of fun because she is not the normal edwardian girl who would be shy. She even drinks beer. So Rose and Jack fall in love. The next morning, Cal is very angry; he treats Rose as his property, so they have a fight. Cal senses that he is going to loose her. Jack and Rose fall in love, though Rose's mother has forbidden her to see Jack again. Rose wants Jack to draw her as she sees his drawings of French girls. After he drew the nude Rose (she only wore the necklace), she puts the drawing and the diamond back into the safe; the two are then persecuted through the ship. The make love in a car, then they are hunted again through the ship. Suddenly the ship crashes into an ice-berg and is slashed open. It sinks though called unsinkable. When Jack and Rose are finally caught, he is suspected of having stolen the necklace. Cal's Butler had put it into Jacks pocket before he was strip-searched. He gets handcuffed and arrested. All passengers struggle for their life, the third class passengers worst. Cal and Jack manage to get Rose and her mother into a lifeboat, but Rose jumps back and rescues Jack; she frees him with an axe from the handcuffs. Finally the ship breaks in two and sinks faster; the infuriated Cal tries to shoot the fleeing Rose and Jack. Cal gets on a lifeboat by pretending to be the father of the child he found and took with him, Rose and Jack stay on the Titanic. They climb over the railing and cling to it. They are not far sucked into the water and so they survive. Rose lays on a drift wood and survives until on of the thirty lifeboat returns but Jack and about 1700 of 2200 passengers die. Rose is brought to another nearby cruiser with destination USA. She sees Cal again on the ship but does not want to talk to him. She does not meet her mother again and so nearly has no money, but she gets an emancipated women and became very old. The exciting thing is that she had the necklace all the time; Cal put it into the pocket of his coat and later gave the coat to Rose and forgot about the necklace. At the end of the film, one can see the lightened Rose who drops the necklace into the water.
A very special thing about the film is the evolution of Rose from a stiffly attired pawn who does everything people tell her to do to a fully realized woman who begins to live life on her own terms. Cameron made her transition not too easy: in one scene she talks back to her mother but then turns compliantly to have her corset straightened.