Antonio, a leading merchant of Venice, is a wealthy, respected, and popular man. Among his many friends is a young man named Bassanio, who owes Antonio a good deal of money. Bassanio would like to repay his friend, but so far he has been unable to do so. However, he now feels that he may have found a way — but he will again need a loan from Antonio. In Belmont, Bassanio tells Antonio, there lives a beautiful and young and wealthy heiress. Bassanio feels sure that he can win her hand in marriage, but he cannot go courting "hands-hanging." If he is to make a good impression, he has to appear at least as well off as her other wealthy suitors. Antonio tells his young friend that he would gladly lend him whatever amount of money he needs, but at the present time he himself is short of cash. All of his money is tied up in his merchant ships, which are still at sea. However, Antonio will not disappoint Bassanio. He knows of a moneylender who will probably lend him the necessary amount, and Bassanio can use Antonio's good name as security for the loan.
At Belmont, Portia speaks to Nerissa, her confidante, telling her how tired she is of the constant stream of suitors, and how she wishes to be free of the perverse obligation of her father's will: Portia cannot choose her own husband; she can marry only the man who chooses the correct one of three caskets — one gold, one silver, and one lead; one contains her portrait and that one is the lucky casket. So far, none of her suitors has decided to risk choosing one of the caskets, which is all for the good, because Portia has no liking for any of them. However, when Nerissa mentions the name of Bassanio, a possible suitor, Portia's mood brightens. He was once a visitor at Belmont, and Portia was impressed with him.
Meanwhile in Venice, Shylock, a rich Jewish moneylender who harbors a secret hatred for Antonio, has agreed to lend Bassanio three thousand ducats for three months, on Antonio's bond. Foregoing his usual high interest rate, Shylock demands instead that if the day for payment falls due and the money is not returned, he may cut off one pound of flesh from Antonio's body. Antonio agrees because all of his ships are due back in Venice a full month before the bond falls due.
A romantic subplot develops when Lorenzo, a close friend of Antonio and Bassanio, falls in love with Shylock's daughter, Jessica. He manages to elope with her by disguising her as a boy, and she manages to take with her a goodly amount of her father's ducats. Of course, this infuriates Shylock, and he vows revenge. Shortly thereafter, Bassanio and Gratiano leave for Belmont, where the "fair Portia" has just sent away the Prince of Morocco and the Prince of Arragon, two more disappointed, unsuccessful suitors. When Bassanio asks to choose one of the caskets, Portia falls immediately in love with him, and she begs him to wait a few days before choosing one of the caskets. He has fallen in love with Portia and insists on taking his chances. He rejects the gold one, then the silver one; he chooses, finally, the lead casket, and on opening it, he finds a portrait of Portia. Both he and Portia are overjoyed, and they make plans to be married at once, along with Nerissa and Gratiano, who have also fallen in love. Happiness reigns in Belmont until Bassanio is brought a letter from Antonio bidding him farewell since his ships have been lost at sea and since it is impossible that he will live after Shylock collects his pound of flesh. Horrified, Bassanio leaves instantly for Venice with money which Portia gives him to pay the bond.
In Venice, Shylock is no longer interested in the mere payment of the money due him. He wants revenge. A Christian stole his daughter (and she took his money), and nothing will satisfy Shylock except the legal fulfillment of the bond. In the court of justice, presided over by the Duke of Venice, Shylock faces his enemy, Antonio. Antonio is surrounded by his friends and is quietly resigned to death. On all sides, Shylock is surrounded by enemies. Bassanio pleads with Shylockto accept double the money due him, but Shylock refuses.
At this point, Portia, disguised as a lawyer, and Nerissa, dressed as her law clerk, enter the court and tell the Duke that they have been sent from Padua by a learned attorney, Doctor Bellario, to plead the defendant's case. Portia entreats Shylock to be merciful, but he will not listen. She offers the moneylender triple the amount owed him, but again Shylock will have none of it. She then solemnly informs the court that Shylock is entirely within his lawful rights. She then informs Shylock that he must be very careful. He must cutoff exactly one pound of flesh, and he must not spill one drop of Antonio's blood. If he fails, all of Shylock's lands and goods will be confiscated. Shylock hastily decides that he will accept the triple payment of the bond, but Portia says no; Shylock then offers to take only the original three thousand ducats, but again Portia refuses, reminding him that it was he himself who demanded the strict interpretation of the law. Furthermore, she says, the law has another hold on him. Since he is an alien in Venice and since he tried to "seek the life" of a Venetian citizen, all his wealth can be divided between the citizen whom he attempted to destroy and the public treasury; in addition, Shylock's own life is in peril because of what he attempted to do.
The Duke decides to spare Shylock's life, but he does give half of Shylock's money to Antonio, and he gives the rest of it to the state. Antonio says that he will not accept the money if Shylock will agree to become a Christian and if, in his will, he will agree to leave his money to his daughter, Jessica, and her new husband, Lorenzo. Shylock, broken and defeated, agrees to all these conditions and leaves the court. Overjoyed, Antonio and his friends offer to pay the young lawyer whatever they can, but, oddly enough, the lawyer wishes only a certain ring which Bassanio is wearing. Bassanio is embarrassed because his wife gave this ring to him and asked him to wear it always. But the lawyer insists and, finally, Bassanio reluctantly gives away Portia's ring. Nerissa likewise cleverly manages to get from Gratiano a ring she gave him. The two ladies then hasten back to Belmont to tease their husbands about the rings.
When Bassanio and Gratiano, along with Antonio, return to Belmont, their wives inquire about the missing rings. Portia and Nerissa insist that the men no doubt gave the rings away to two other women. The husbands swear that it is not true, and it is not until Portia and Nerissa have put their husbands through some long, comically agonizing moments of discomfort that they confess that they themselves were the "learned doctor" and the "clerk" to whom the rings were given. Thus all ends happily, as Portia gives Antonio a letter informing him that three of his ships have arrived safely in port.