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In 1996, aboard the research vessel Akademik Mstislav NAME_1 and his team search the wreck of RMS Titanic. They recover a safe they hope contains a necklace with a large diamond known as the Heart of the Ocean. Instead, they find only a drawing of a young nude woman wearing the necklace. The sketch is dated April 14, 1912, the same day the Titanic struck the iceberg that caused it to sink.[Note 2] After viewing a television news story about the discovery, centenarian NAME_2 contacts NAME_3, identifying herself as the woman in the drawing. Hoping she can help locate the necklace, NAME_3 brings NAME_4 aboard Keldysh, where she recounts her experiences as a Titanic passenger.

In 1912 Southampton, 17-year-old NAME_5 NAME_6, her wealthy fiancé NAME_7 NAME_8 and NAME_4's widowed mother NAME_9 board the Titanic. NAME_9 emphasizes that NAME_4's marrying NAME_10 will resolve the family's financial problems and maintain their upper-class status. Meanwhile, NAME_11, a poor young artist, wins a third-class Titanic ticket in a poker game. After setting sail, NAME_4, distraught over her loveless engagement, climbs over the stern railing, intending to commit suicide. NAME_12 coaxes her back onto the deck and they develop a friendship. NAME_12 soon admits that he has feelings for NAME_4. When NAME_10 and NAME_9 object, NAME_4 rejects NAME_12's attentions, but returns to him after realizing she has fallen in love.

NAME_4 brings NAME_12 to her state room and requests he draw her nude, wearing only the Heart of the Ocean. They later evade NAME_10's servant, NAME_13, and have sex in a Renault Towncar inside the cargo hold. On the forward deck, they witness the ship's collision with an iceberg and overhear its officers discussing its seriousness. NAME_10 discovers NAME_12's sketch and an insulting note from NAME_4 in his safe, along with the necklace. When NAME_12 and NAME_4 return to warn the others about the collision, NAME_10 has NAME_13 slip the necklace into NAME_12's pocket to frame him for theft. NAME_12 is confined in the master-at-arms' office. NAME_10 puts the necklace into his own overcoat pocket.

With the ship sinking, the crew prioritize women and children for evacuation. NAME_4 finds and frees NAME_12, and they make it back to the deck, where NAME_10 and NAME_12 urge NAME_4 to board a lifeboat. Intending to save himself, NAME_10 lies that he will get NAME_12 safely off the ship and wraps his overcoat around NAME_4. As her lifeboat is lowered, NAME_4, unable to abandon NAME_12, jumps back onto the ship. NAME_10 grabs Lovejo