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he wants to marry portia, so he needs money
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Bassanio has no money and so his friend Antonio borrows the money from Shylock.
Shylock doesn't like Antonio so agrees to lend him the money without any interest on the security of a pound of flesh from anywhere on Antonio's body. Basically if Antonio doesn't pay up, Shylock can kill him. -
Bassanio, with the aid of the money, goes to woo Portia. She loves him alright but, according to her father, she must marry the man who can guess which of the three boxes her picture is in. Bassanio guesses the right box, so they get married.
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But watch out. Antonio is unexpectedly he cannot pay Shylock. Shylock takes him to court and starts sharpening his knife.
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Portia disguises herself as a lawyer, and gets Antonio off on a technicality. She convinces the court to order Shylock to give all his money to his daughter and to her husband and to stop being Jewish.
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Portia asks Bassanio for a share of her services and insists on taking his wedding ring, which he swore to her he would never get away. Antonio persuades him to do this, and of course Antonio didn't let him talk when he gets home to his new wife without his wedding ring.