Although contrary to actually how Titanic was led to the sinking, the movie brings out the story on different lines.
The brash egotistic owner of the ship demands speeding the ship to occupy the headlines in the newspapers the next day to a reluctant captain. The captain retires for the day after giving his orders, but as fate might have it, an iceberg is spotted by the lookouts only after that. The ship was already speeding, and by the time information reached the captain, and fresh orders could be made, it was too late! The iceberg does the damage to some of the watertight compartments below, and it becomes evident - the ship was going to sink. Hardly prepared for such a situation, there is chaos amongst everyone - passengers and crew members alike! There seems to be shortage of lifeboats, and women and children end up getting priority where everyone is dying to escape.
Rose escapes with Jack to the deck of the ship after having a nude portait of hers done by Jack, with her wearing the diamond necklace that Cal had persented her for their engagement and the two end up making love in the ship's cargo hold. Soon afterwards, they discover the catastrophe that is to befall them. An infuriated Cal, from seeing the portrait, gets Jack to be arrested by accusing him of stealing the diamond. A quick drama of how Rose escapes her company and saves Jack follows. As the two reach the top deck, both Jack and Cal persuade her to aboard the life boat, promising they have been arranged for one too. As soon as she finds out that Jack is going to be left behind, she jumps back on board the Titanic. In all this, Cal realises that Rose now has the diamond necklace, charges down after them with his pistol, but decides to give up on the chase and jumps aboard one of the lifeboats pretending to be a woman.
The Climax - Jack and Rose are at the very end of the bow, and awaiting the end that seemed quite certain. The ship breaks in half, and the stern side rises 90 degrees into the air. The two hold on tightly to each other, Jack reassures her that they were going to live through this. They ride into the ocean with the stern, and soon after, Jack helps Rose onto a wooden panel. Unfortunately, the panel couldn't support Jack, so Jack remains in water talking to Rose, reassuring her every moment that she will live through all this and die an old woman.
Rose, who was motionless for quite a while, suddenly wakes up to find Jack motionless - he succumbs to the cold. To her luck, she finds a lifeboat that comes in search of survivors but doesn't spot her. She blows the whistle to catch their attention and gets rescued. Back in New York via RMS Carpathia, she prefers to live a free life as Jack had always wanted her to, gives her name as Rose Dawson, hides away from Cal and sets foot into the mainland.
The story thus ends with the aged Rose telling this story to the treasure hunters, and ends up dropping the 'Heart of the Ocean' back to where it belonged and retires to bed with a lighter heart and falls into an eternal sleep, and as the last survivor her spirit returns to the Grand Staircase on board a magical and restored to her former glory Titanic where Jack, and the other souls, have been patiently waiting for her to return home.